For the Sponge Cake: Preheat oven to 325° F. Line the bottom of a 10 springform pan with parchment paper and lightly grease the sides of the pan.

Beat eggs, sugar, salt and vanilla on the highest speed of an electric mixer for 8 minutes, until very light and airy. Sift flour over the egg mixture in two batches and gently fold it in until there are no visible streaks of flour. Spread batter in pan and bake for 20 minutes, until golden brown and the top springs back when gently pressed with a finger. Cool in pan on a wire rack.

For the Topfen Cream Filling: Beat the whipping cream until stiff and set aside. In a large mixing bowl, beat the topfen, sugar, vanilla, lemon zest and juice until smooth.

Place the gelatin leaves in a bowl of cold water to soften for 5 minutes. Remove the gelatin from the water and gently dissolve it in a small saucepan over low heat. Remove from heat and add 6 tablespoons of the topfen cream to the gelatin and stir until completely smooth. Whisk the gelatin mixture into the remaining topfen cream. Fold in the whipped cream.

Assembly: Split cake horizontally. Line inside of cake ring or springform with parchment or plastic wrap. Place the bottom layer on a platter, cut side up and tighten cake ring around it.

Spread the topfen filling over the cake, place the top cake layer on top and press it down gently. Cover tightly with plastic wrap and chill for at least 4 hours, preferably overnight.

Release ring from cake, dust with powdered sugar and garnish with berries and mint leaves.

Stored tightly covered. Will stay fresh for 3 days in the refrigerator.