Preheat oven to 320 °F (160°C). Line the bottom and the sides of a 9" (23cm) springform pan with parchment paper and set aside. The easiest way to line a springform pan with parchment paper is to cut out a circle for the bottom and 2-3 inch strips for the sides.
In a large mixing bowl, using a stand or handheld mixer fitted with a whisk or paddle attachment, mix eggs, sugar, butter, and vanilla* until creamy and fully combined about 2-3 minutes.
Sift in flour, cocoa, baking powder, and salt. Stir to combine, another 1-2 minutes. Transfer to the prepared baking pan and bake for about 33-38 minutes or until a toothpick in the center comes out clean. Do not overbake. I baked mine exactly 35 minutes. Let cool to room temperature.
Place drained sour cherries into a medium-sized bowl. Set aside.
In a small bowl, stir together cornstarch and 4 tablespoon of canned sour cherry water to combine.
In a heavy-bottomed saucepan, bring ¼ cup canned sour cherry water, kirsch, and sugar over medium heat to a simmer. Stir constantly. Pour in the cherry-cornstarch mixture and cook for 1-2 minutes until it gets thick like syrup and coats the back of a spoon. Stir continuously. Remove from the heat and pour over sour cherries. Let cool to room temperature.
In a large mixing bowl, whisk heavy whipping cream until soft peaks form. Add sugar and whisk until stiff peaks form.
Cut the cake horizontally two times that you have three almost evenly thick cake layers. Place the bottom cake layer on a cake stand or plate and brush with cherry syrup lightly.
Spread with whipped cream and pipe an additional ring of whipped cream around the edges (together about ⅙ of the cream). Watch the photos and video for better understanding. Then spoon the half of the sour cherries (with very little liquid) on top of the cream but inside the piped ring. The ring makes sure that the cherries stay inside the cake and no syrup is running out the sides. Then spoon additional whipped cream on top of cherries and spread evenly to cover them (another ⅙ of the cream, so that about ⅓ of the cream is now used in total). Place the second cake layer on top and repeat.
Finish with the third cake layer on top and spread remaining ⅓ of whipped cream all over the cake and around the edges. Chill in the fridge for at least 4 hours.
Before serving, decorate with chocolate sprinkles, fresh cherries, and additional whipped cream to your preference. Store in an airtight container in the fridge for up to 3 days.