Step 1

Preheat the oven to 350°F (180°C) and place the rack in the lower third of the oven. Grease the inside of a 9-inch springform pan. Line the bottom with a parchment paper round and grease the paper. Set aside.

Step 2

In a large mixing bowl combine the flour, sugar, and salt. Add the butter and the egg.

Step 3

Stir to combine and press the dough together until it forms a smooth ball. Wrap the dough in plastic wrap and chill for 30 minutes in the refrigerator while you prepare the filling.

Step 4

In the bowl of an electric mixer fitted with the paddle attachment, beat together the butter and sugar until smooth and creamy, about 1 to 2 minutes. Beat in the egg yolks, then the cornstarch, vanilla extract, and lemon zest, if using, and beat to combine.

Step 5

Add the quark and heavy cream, and beat until smooth. In a separate mixing bowl, beat the egg whites to stiff peaks (by hand with a whisk or use your electric mixer with the whisk attachment). This will take about 2 to 3 minutes with an electric stand mixer at medium-high speed. Fold the beaten egg whites into the cheesecake batter.

Step 6

Take the chilled dough out of the refrigerator. Break it into smaller pieces and press it into the bottom and sides of the prepared springform pan.

Step 7

Pour the prepared filling into the crust. Transfer the pan to the rack on the lower third of the oven, and bake for 60 minutes.

Step 8

Then crack open the oven door and let it sit in there for another 10 to 15 minutes. The edges should be browned and the center will wobble a bit if you gently shake it, but will set as it cools. The center will also start to settle and sink as it cools.

Step 9

Remove from the oven and carefully remove the edges of the springform pan (if greased properly they should come right off). Cool completely on its base at room temperature. Transfer the käsekuchen off the springform pan base (peel off the parchment paper as well) onto a serving dish.

Step 10

Dust the top with powdered sugar and serve with fresh lightly sweetened whipped cream (schlagsahne) and berries, if desired.