Bake the chocolate sponge cake according to the package instructions. Allow it to cool completely on a wire rack, then slice into 1-inch cubes.

In a medium saucepan, combine cherry syrup, granulated sugar, and cornstarch. Cook over medium heat for 5-7 minutes until thickened. Stir in drained tart cherries and kirsch, then remove from heat.

In a separate saucepan, combine whole milk, sugar, cocoa powder, and cornstarch. Heat gently, whisking until warm. Temper the egg yolks, then combine everything back into the saucepan. Cook for 8-10 minutes until thickened.

In a large mixing bowl, beat heavy cream, powdered sugar, and vanilla extract using a hand mixer until stiff peaks form.

In a trifle dish, layer chocolate cake cubes, drizzle with kirsch, add chocolate custard, cherry compote, and whipped cream. Repeat layers until finished.

Cover with plastic wrap and refrigerate for at least 4 hours or overnight.

Before serving, garnish with extra whipped cream, chocolate shavings, and fresh cherries.