1. PREHEAT THE OVEN
2. Preheat the oven to 180°C - 356 F. Grease and flour a 9 inch springform pan (24 cm) with baking spray.

Make the pastry

1. The secret for a successful German Mürbeteig is to use cold ingredients and cool hands. Don't knead the dough too long, it might get crumbly. Sift the flour into a bowl and mix with baking powder, make a hollow like in the picture above.
2. Add the sugar and the egg into the hollow. Cut the cold butter into pieces and put it on the rim.
3. Mix the egg and sugar with your fingertips, then quickly rub butter into the flour and blend all ingredients together. Don't knead too long.
4. Form a ball with the dough, wrap in cling film and let it rest in the fridge for at least half an hour.
5. Quickly knead the dough again, roll it out on a floured table.

FILLING

1. If using frozen cherries, defrost the cherries, reserving their juice. Strain the defrosted cherries and measure the juice. Add enough tart cherry juice to make one generous cup
2. If using canned, strain the cherries. Measure out 250-280ml of tart cherry juice - so about one whole cup plus 2 tablespoons and mix the juice with the cornstarch, sugar and, if using, bitter almond extract in a large saucepan with a whisk. Bring the mixture to a boil and add in the cherries. If using kirsch, add it at this point. Mix everything together with a wooden spoon and set the sour cherry filling aside to cool.
3. Streusel topping
4. In a small bowl combine all the streusel ingredients. Rub with your fingers until the butter is well-incorporated and the mixture is crumbly. Set aside in the fridge to cool.

Assemble

1. Transfer the shortcrust dough into the pan. You can roll it out and transfer it, then or use your fingers to press the dough evenly over the bottom of the pan and about ⅔ of the way up the sides of the pan. Use the bottom of a small measuring cup or glass to push into the dough against the sides all the way around to ensure your side-crust and corner aren't too thick. Push down against the top of the side-crust of dough with your fingertips to make it even all the way around. Prick with a fork.
2. Pour the partially cooled sour cherry filling over the base. Sprinkle the streusel crumbs evenly over the cherry filling.
3. Bake it: Bake the cake in the preheated oven for 35 to 40 minutes, or until the streusel is golden brown on top.
4. Let the cake cool in the pan until completely cold. You can even make the cake ahead of time up until this point and let it sit on the counter, uncovered and in the pan, until serving it the next day.
5. Top it: Before serving, whip the cream with the vanilla, whip it and sugar until of spreading consistency. Add the vanilla and, if you want, some Kirsch and whip for a few seconds more to incorporate.
6. Remove the sides of the springform pan and set the cake onto a cake platter (you can leave the base layer underneath if you wish, or carefully use two spatulas to transfer the cake off the pan base and onto a plate).
7. Spread or pipe the whipped cream over the top of the streusel layer, bringing it as close to the edges as possible. Use a fine-meshed sieve to sprinkle a dusting of unsweetened cocoa powder over the top of the whipped cream