Heat oven to 190c/fan 170c/gas 5. Butter 2 x 20cm sandwich tins and line each with a circle of baking parchment. In a large bowl, beat together all the cake ingredients until you have a smooth, soft mixture. Spoon the mixture equally into the two tins, smoothing over the top of each with the back of the spoon. Bake in the oven for 20 mins until golden and the cake springs back when gently pressed. Turn the cakes onto a cooling rack.
Heat the sugar, 2 tbsp water and Disaronno together until the sugar has dissolved. Leave to cool, about 10 mins. Use a large serrated knife to cut each cake in half. Brush the syrup all over all four pieces of cake with a pastry brush.
For the filling, whip the cream until it forms soft peaks. Beat the mascarpone and caster sugar in a large bowl to loosen, then fold in the cream and mix together until smooth.
Spoon a third of the cream mixture over one of the cake halves. Scatter over some of the fruit (you don’t want to cover the cake), then sandwich another half on top. Spread with cream and fruit as before and top with another half of cake and more cream and berries. Lay the final cake half on top. Gently press down, then wrap tightly in cling film and leave in the fridge overnight. To stop the cream from melting, this cake needs to be transported to the picnic in a cooler box. At the picnic, carefully remove clingfilm and dust with icing sugar to decorate, if you like.