Pre-heat the oven to 180°C.
Grate the marzipan and whisk together in a bowl with the egg whites and brown sugar.
Spread the mixture out on a parchment-lined baking sheet, about 20x30 cm.
Bake at the bottom of the oven for 10-12 minutes, depending on how sticky you want the base. The shorter the time the stickier the base. Leave to cool.
Prepare the butter cream. Heat the blackberries in a pan while stirring. Then sieve the blackberry purée into a bowl so that the pips are left behind. Leave the purée to cool.
Whisk the egg yolks and icing sugar together in a bowl. Add the softened butter a little at a time and whisk for about 10 minutes. Add the blackberry purée and whisk until the mixture is smooth and fluffy. Leave the mixture to stand in the fridge for 20 minutes.
Spread the butter cream evenly over the base. Place in the freezer, and melt the chocolate over a water bath.
Spread the chocolate over the butter cream and place in the fridge. Cut the cake into small pieces before serving.
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