Using a stand mixer with paddle attachment or large mixing bowl and hand mixer, whip the butter until creamy then add the sugar and salt. Beat until combined. Add the egg and beat again until combined. Mix 250g of the flour with the baking powder together then gradually add them and beat until combined. Wrap the dough and chill it in the fridge for at least 1 hour or overnight.

Put the quark in cheesecloth over a bowl or in a large paper coffee filter for 1 hour to drain the excess liquid. Set aside. Pour the canned apricots over a sieve and let the excess liquid drain off. Set aside.

Preheat your oven to 375 degrees F and grease the bottom and edges of a 26cm spring form pan. Flatten the dough into a disk with your hands and cut away 1/8 of it. Wrap and place this 1/8 back in the fridge to stay cool. Cut away another ¼ of the dough and set aside. Generously dust a work surface with flour and roll out the remaining dough so it is slightly larger than the bottom of the spring form pan (no thinner than a ¼ inch). Lay the bottom of the spring form pan on top, cut away the excess dough and set the excess dough aside with the other 1/4 dough. Lay the rolled out disk of dough into the bottom of the spring form pan. Gently press the dough out to the edges with your fingers.

Combine the remaining ¼ dough and excess that was trimmed off and roll it out into a long tube the same length as the inside diameter of the spring form pan. Take the tube and place it along the bottom inside edge of the pan. Press the dough down to connect with the bottom layer and up the edges approximately 2 inches high. Set aside.

Using a stand mixer with whisk attachment or large mixing bowl and hand mixer, beat the three egg whites until stiff. Set aside. Beat the three egg yolks, sugar, quark, lemon zest and juice, cornstarch and vanilla extract until combined. Gently fold in the egg whites with a large spatula until just combined. Pour the mixture into the spring form pan and spread evenly.

Place the apricot halves cut side down side by side on top of the mixture to fill the pan. Bake for 35 minutes. While it is baking, mix the 1/8 chilled dough with the remaining 30g of flour until it resembles course crumbs. Place this mixture in the fridge to chill while the cake bakes. After 35 minutes, remove the cake from the oven and sprinkle this mixture on top. Return the cake to the oven and bake for an additional 25 minutes or until center is set and top is golden brown.

Let the cake cool for a few hours, then chill in the fridge before serving. Optional: dust finished cake with icing sugar before serving.