beat the ingredients
Place a large pot filled with 200 ml or 1 cup of water over medium-low heat. Place a large heatproof bowl over the pot. Add eggs, sugar, salt, and vanilla essence to the bowl. Using an electric mixer, beat the mixture for about 5 - 8 minutes or until tripled in size and pale.TipIt is very important to beat the mixture for long enough. The temperature of the egg mixture should be warm and reach about 45 - 50 °C or 110 - 120 °F.
Remove the egg mixture from the heat. While mixing with an electric mixer, start adding warm, freshly brewed espresso coffee and melted butter. Mix to combine, then add flour, unsweetened cacao powder, and baking powder. Mix until combined, then gently stir everything using a spatula.
bake the sponge cake
Grease a 20 cm / 8-inch springform cake pan. Brush with butter generously on the entire inside of the pan. Cut a piece of baking paper to fit the bottom of the pan and place it on the bottom of your greased cake pan. Pour the cake mixture into your prepared cake pan. Place in the preheated oven. Bake for 45 - 50 minutes at 170 °C / 340 °F.
swiss meringue espresso buttercream
Place a large pot filled with 200 ml or 1 cup of water over medium-low heat. Place a large heatproof bowl over the pot. Add egg whites and sugar to a bowl. While regularly whisking with a whisk, cook the mixture until it will be hot on touch and it reaches 55 - 60 °C or 130 - 140 °F. It will take approx. 10 - 12 minutes. Remove the egg whites from the heat and mix into a thick meringue. Mix with an electric mixer for approx. 10 minutes. The meringue will start to cool. When it reaches room temperature, start adding cold butter cubes while constantly mixing with an electric mixer. Only add the next cube of butter once the previous one has been completely incorporated. Now add the cold espresso coffee. Mix until the mixture becomes smooth and creamy. The mixture will separate while mixing, but do not stop mixing. Continue mixing at the highest speed until creamy and smooth.TipIf the butter melts completely when you add it to the meringue, then the meringue is still too hot. You first need to cool it, only then start adding the rest of the butter.You can prepare the espresso buttercream for up to 3 days in advance. Keep it chilled in the fridge. Bring to room temperature 2 hours before using it. Mix well with an electric mixer.If the buttercream is too runny, place it in the fridge for 20 - 30 minutes. Then mix again with your electric mixer. The buttercream will separate but do not worry, continue mixing until creamy and smooth.
Turn the baked sponge cake upside-down on a wire rack to cool completely. The cake if flatten. When the sponge cake is cold, spread the espresso buttercream on top. Optionally sprinkle with hazelnut crocant. Slice and serve.