We've reached the second last week of Bake Off, and the judging is fierce. The four best bakers are in the semi-finals, and even a small mistake could be the difference between making it to the finals and going home.

Signature Bake: Baked Cheesecake

There have been some great challenges this season (bagels!), and a baked cheesecake is the perfect signature challenge for the semi-finals. The bakers have two and a half hours, and they need to make their cheesecakes spectacular, which is no longer a criterion exclusive to the showstopper. We're at the pointy end, people. Mel tells us that the earliest recording of a baked cheesecake was in Ancient Greece! Maybe there should be a time travel week next season. Ancient bakes? I'll work on it.

Antonio is making an Italian ricotta, vanilla and citrus cheesecake with a side of vanilla bean ice cream. Matt asks how he's going to make it special - that's what the ice cream is for! The ice cream is delicious, and Maggie offers Antonio a job, but the cheesecake is slightly overcooked. Only a little bit, but this is semi-finals, so the judges are being more critical than they would have been in week 1.

Olivia is making a black forest cheesecake, which Maggie deduces from the ingredients on her bench. The judges are looking for something extra, and Maggie asks Olivia how she'll make it special. Olivia replies that she's making a cherry tree to go on top as decoration. The cake itself will have a chocolate biscuit base and a chocolate and cherry filling. It's so good - Matt even uses the word 'superb' - that the judges forgive the cracks on top, and Olivia has made the best cheesecake for the day.

Monica's cheesecake has a pre-bought biscuit base so that she has enough time to make the three layers of her Neopolitan cheesecake. That's how I make cheesecake bases, but it's the semi-finals, so Maggie and Matt are probably expecting the contestants to make their own bases. The layers for Monica's cheesecake are milk chocolate on the bottom, vanilla and white chocolate in the middle, and strawberry on top. The strawberry is the best of the three, followed by the milk chocolate layer, but the white chocolate didn't work out as well.

Liesel is making a German cheesecake. She found the recipe on the back of a letter that her aunt wrote to her mum, which is great. The cheesecake has a shortcut pastry base, ricotta filling, and is topped with sour cherries and vienna kipfel, aka almond shortbread. The flavours are great, but the judges think it's more of a tart than a cheesecake.

Technical Challenge: Porcini and three cheese soufflé

This is a Maggie Beer recipe, and before she leaves she tells the bakers that burnt butter is better. As this is a soufflé challenge, the bakers will have staggered start times, so Maggie and Matt can taste their bakes when they're fresh. Olivia is the first to start, and Monica, Liesel and Antonio leave the shed before Olivia hears what she will be making. The rest of the bakers find out what they’ll be making when they begin the challenge at five minute intervals. The trickiest part of this challenge is waiting. The soufflé needs to be in the oven for 40 minutes, but you can't open the oven to see how it's going because it will deflate.

Olivia's soufflé is the first to be tasted, and it's risen well. It's golden on the top and Maggie says the mixture is perfect. Olivia watches on nervously because she can't hear what the judges are saying. Antonio's soufflé is golden, but it hasn't risen as much as Olivia's. The flavours are all there, but unfortunately it's a little overcooked. They love Monica's, and Liesel's had some problems with her crumb.

From fourth to first: Antonio, Liesel, Olivia and Monica. The judges tell Olivia that Monica's was slightly better, which is a demonstration of how close this competition is.

Showstopper: Around the World cake

For the penultimate showstopper, the judges have left a lot of room for interpretation of the brief. The contestants are asked to draw inspiration from their heritage, their travels, and people that have inspired them. Maggie says they need to pull out all the stops to make it to the final.

Antonio has abandoned his Italian heritage to make a French cake. His Gateau St Honore has alternating layers of choux buns and puff pastry, a lychee and a raspberry creme patissiere and rings of tempered white chocolate. Matt says the profiteroles are some of the best he's ever tasted on the show, but Maggie says the puff pastry is a little undercooked. She said she could just sit there and keep eating it though, so this will be a tough challenge to judge. After Antonio takes his cake back to the bench, Maggie and Matt are still talking about his flavours, which is a very good sign.

Olivia has German heritage, but her family moved to Australia a generation before Liesel's. For her showstopper, Olivia is making a Schichtortte (a great GBBO series 5 technical challenge), which is a cake baked in thin layers. Olivia's cake will have 40 alternating layers of orange and almond, and will be decorated with a chocolate glaze and sugar work. Her cakes are moist, and the decoration is stunning. Matt counts all the layers in the cake just to make Olivia nervous. Maggie loves the almond and orange flavours together, and it looks as if Olivia will be this week's Star Baker.

Liesel is also making a Schichtortte! She knows she's going to be judged directly against Olivia, so she's understandably nervous. Liesel's Schichtortte will have 20 layers, some of which will be apricot jam. She's decorating the cake with a mirror glaze and tempered chocolate. The presentation is simple, but the work went into baking the cake. There are clearly defined layers, but it's unfortunately a little dry. Maggie suggests that she add jam to more layers of the cake, which would solve that problem.

Monica is making a Pomme Charlotte. With lady fingers for decoration, the cake will have alternating layers of sponge and apple bavarois. Matt doesn't want anyone to have an easy time today, so he tells Monica that Pomme Charlotte is one of the first things he made in a kitchen. Monica had fun with her decorations, but Matt thinks it's a bit much. It tastes great though, and the apple isn't too sweet.

Olivia wins Star Baker, and Mel pauses just long enough to make me doubt my instincts. She tells us that she really wants to win. Of course you do! Liesel is going home, and it's tough to just miss out on the final. The final shot of the week is Antonio calling his Nonna to tell her he made it to the finale, it's gorgeous.

For recipes from this week’s episode, visit the show’s website.