Preheat the oven to 350°F. Pour 1½ cups of milk over the gingerbread cookie crumbs and let everything soak.
Beat the eggs and sugar until frothy. Mix the flour, baking powder, and cocoa powder together and add to the egg mixture, along with ⅓ cup milk.
Add the soaked gingerbread crumbs to the batter and mix everything until combined. Divide the batter between 2 greased springform pans and bake for about 60-70 minutes.
For the frosting, first crush leftover Christmas cookies in a food processor. Beat the softened butter with a hand mixer until fluffy and gradually add in the powdered sugar. Finally, mix in the cream cheese and the cookie crumbs.
Cut off the tops of the cooled cakes to create smooth lines. Spread some frosting over the first cake, place the second cake on top, and cover the entire cake with frosting. Make sure to save some of the frosting for decoration later on.
Melt the leftover Christmas chocolate over a hot water bath and stir in the heavy cream to make a smooth chocolate ganache. Once the ganache has cooled a bit, pour it over the cake, making sure it also dribbles down the sides of the cake.
Spoon the rest of the frosting into a piping bag with a star nozzle and pipe small rosettes on top of the cake. Sprinkle chocolate shavings over the cake and decorate with some more leftover Christmas treats.