Cut butter into pieces, melt at low heat and let it cool down again. Cream eggs and 75 g sugar. Add lemon zest. Mix flour and baking powder. Gradually stir in the liquid butter and flour.
Spread the dough evenly on the greased glass sheet (30x26 cm) sprinkled with breadcrumbs. Place in the cold oven on the middle shelf and bake at 200 °C/ gas mark 3 for approx. 20 minutes.
In the meantime, mix 125 g sugar and approx. 6 tablespoons of lemon juice until the sugar is completely dissolved. Stick many holes into the hot cake with a wooden skewer and sprinkle with lemon juice.
Let it cool down. Put the peaches briefly in boiling water, rinse under cold water and peel off the skin. Halve the peaches, remove the stone and cut the halves into slices. Drizzle peaches with remaining lemon juice.
Select the blueberries, wash and drain well. Put the cream and sour cream into a mixing bowl and whisk until stiff with the whisk of the hand mixer. Add vanilla sugar, cream setting agent and remaining sugar.
Spread the sour cream loosely on the cake and generously cover with peaches and blueberries. Sprinkle with pistachios and keep cool until serving. Decorate as desired with lemon balm leaves.
Results in about 16 pieces.