Preheat the oven to 180°C, melt the white chocolate in a double boiler and the butter on a very low heat or in the microwave.
Beat the eggs and the sugar until the mixture becomes light and fluffy. Pour in the melted butter and white chocolate and whip the batter.
Add the flour, the baking powder, the sieved lemon juice and the lemon peel. Stir until everything is evenly mixed.
Pour into a rectangular cake tin, previously greased and floured. Bake for about 1 hour until golden and dry in the centre (use a toothpick to check). Let it cool down completely before unmould.
Make the frosting: whisk the mascarpone cheese with the whipped cream, 2 tablespoons of icing sugar and the finely grated zest of a lemon. Spread the mascarpone frosting over the cake, then decorate with plenty of strawberries and cherries and a few mint leaves. Serve immediately or store the cake in a cool place until ready to serve it.