Preheat the oven to 200 degrees Celsius/ 390 degrees Fahrenheit. Line a baking tray with baking paper.
Beat egg whites: Separate the eggs. Beat the egg whites until stiff. Set aside.
Beat egg yolks: In another bowl beat the egg yolks and the warm water with 100 g/ 3.5 oz/ ½ cup sugar until pale and fluffy, about 6-7 minutes.
Mix the flour, cornstarch, and baking powder. Mix them into the egg yolk mixture. Carefully incorporate the stiff egg whites as well.
Bake: Pour the batter into the prepared tray and bake in the preheated oven for about 15 minutes or until golden.
Coffee: In the meantime, brew the coffee. Using a tablespoon distribute the coffee evenly over the hot cake base and let the cake cool completely.
Topping: Mix the quark, lemon juice, and the remaining 50 g/ 1.8 oz/ ¼ cup sugar. You may add a bit more sugar if you find it necessary.
Whip the heavy cream until stiff and carefully fold it into the quark mixture. Spread the mixture over the cooled cake.
Top with fresh blackberries.