Mix flour and baking powder. Wash lemon hot, grate dry, grate peel thinly, halve fruit and squeeze juice. Cream butter and sugar with the whisk of the hand mixer. Stir in eggs one after the other. Add mascarpone, flour mixture, lemon juice and zest and mix to a smooth dough
Fill the dough into a greased, flour-spread box form (10 x 30 cm) and smooth it down. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 1 hour
Remove the pistachios from their shells. Stir icing sugar and 3 tablespoons of water until smooth. Take the finished cake out of the oven, let it rest in the mould for about 10 minutes, turn out of the mould and let it cool down. Spread the icing on the cake and sprinkle with pistachios
Waiting time approx. 1 hour