Preheat the oven to 180 C / Gas 4. Grease two 18cm cake tins and line with baking parchment.
First make the sponge cakes: In a mixing bowl, beat 3 of the eggs with 100g sugar until creamy. Add the flour and baking powder and mix well. Divide equally between the two prepared cake tins.
Bake in the preheated oven for 8 to 10 minutes or until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool. Once cool, cut into slices to form the layers.
In the meantime, separate the remaining eggs into two mixing bowls. First whisk the egg whites until soft peaks form, then gradually whisk in 100g of the remaining sugar. Whisk until glossy, stiff peaks form.
In the second bowl, beat the egg yolks with the remaining sugar then mix in the mascarpone cheese and vanilla extract. Mix until smooth and well blended. Combine egg whites and yolks and mix well.
Pour the coffee into a shallow dish then dip the prepared cake slices into the coffee. Lay in the base of a glass dessert or trifle dish or a springform cake tin.
Top the base layer of sponge with half the mascarpone mixture. Dust with 2 tablespoons of cocoa, using a sieve to achieve an even layer.
Place the second layer on top, also soaked in coffee. Top with the remainder of the mascarpone mix and smooth the top. Dust the top with a little cocoa powder.
Chill in the fridge for at least two hours to set. Once set, slice and serve.