Sernik - Polish Cheesecake

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1. 1Step 1: Stir the flour, cocoa powder, sugar, baking powder, and salt together.
2. 2Step 2: Add the egg yolk, sour cream, vanilla extract, and cold butter (cut into small cubes).
3. 3Step 3: Rub the ingredients between your fingers or mix with a stand mixer fitted with a paddle attachment until the dough comes together. It will look crumbly at first but it will soon form a smooth dough.
4. 4Step 4: Divide the dough into two parts. One part should be slightly bigger than the other part (about 3 Tbsp), flatten each part into a flat disc, wrap in plastic foil, and put in a fridge for 45 minutes.
5. 5Step 5: Preheat the oven to 350°F (175°C), Gas Mark 4, no fan.
6. 6Step 6: Grease a 10-inch (26cm) springform pan with butter (grease the sides generously) and line the bottom of the pan with parchment paper.
7. 7Step 7: Transfer the smaller part of the crust to the freezer. Take the larger part of the crust and tear it into pieces (or cut with a knife if it's too firm). Flatten the pieces and press them into the pan to cover its bottom.
8. 8Step 8: Prick the crust with a fork several times.
9. 9Step 9: Bake the crust for 20 minutes.
10. 10Step 10: While the crust is baking make the cheesecake batter.
11. 11Step 11: Process the cheese in a food processor until smooth (it doesn't have to be completely smooth, a couple of small lumps are fine). Alternatively, you can put the cheese two times through a meat grinder (use a plate with small holes).
12. 12Step 12: Separate the eggs into egg yolks and egg whites. Make sure there is no egg yolk in the egg whites or you won't be able to beat them properly.
13. 13Step 13: Beat the egg yolks with soft butter, 1/2 of the sugar, and orange zest until combined. Use an electric mixer or a stand mixer fitted with a whisk attachment.
14. 14Step 14: Add the cheese and vanilla extract, beat until combined.
15. 15Step 15: In a separate, very large and clean bowl, beat the egg whites (with a clean whisk attachment) until soft peaks form. Slowly add the remaining sugar and beat the egg whites until stiff. Add the corn starch and beat until combined.
16. 16Step 16: Add about 1/4 of the beaten egg whites to the cheese batter, fold in with a spatula until combined.
17. 17Step 17: Add the cheese batter to the remaining egg whites. Fold in gently with a spatula until combined and there are no white strikes of egg whites.
18. 18Step 18: Pour the cheese batter onto the warm, baked crust. The batter will reach almost to the rim of the pan but don't worry, it won't overflow.
19. 19Step 19: Take the second part of the crust out of the freezer and grate it on the large holes of a box grater. Sprinkle the crust onto the cheesecake batter.
20. 20Step 20: Bake the cheesecake for 1 hour to 1 hour and 10 minutes. Turn off the oven and open the oven door slightly. Leave the cheesecake in the oven for 10 minutes then take it out and place on a cooling rack. It will be slightly jiggly in the middle but set. The cheesecake will rise and crack in the middle and then it will fall - it's just the nature of this cheesecake.
21. 21Step 21: Leave the cheesecake on the cooling rack to cool completely (in the springform pan), at least 3 hours (do not try to cut warm cheesecake) but preferably overnight (it has a better texture and flavor).
22. 22Step 22: Enjoy!