Wash lemons hot, rub dry. Grate the zest of 1 1/2 lemons thinly. Halve lemons, squeeze juice. Set aside 1 squeezed lemon half with its peel. Measure 75 ml juice.
Beat the fat, 185 g sugar, salt, vanillin sugar and lemon zest until light and creamy with the whisks of the hand mixer. Stir in the eggs one after the other. Mix the flour and baking powder and stir in the lemon juice alternately. Pour the dough into a greased, flour-spread box form (11 x 25 cm), smooth it down and bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 45-55 minutes (stick test). Cover the cake after about half the baking time. Remove from the oven and allow to cool for approx. 20 minutes. Turn the cake out of the tin and let it cool down on a cake rack.
Whisk the curd, mascarpone and 2 tbsp. sugar with the whisk of the hand mixer until thick and creamy. Spread the cream loosely over the cake, chill for approx. 30 minutes. Sort the berries. Spread the lavender and berries over the cake and chill until serving. Rub lemon zest over the cake before serving.