Line a 2½-quart bowl generously with plastic wrap, leaving plenty of overhang to cover the top later. Cut the sponge cake in half—using a cake slicer makes this easier. Place one half of the cake at the bottom of the bowl. If using a loaf cake, slice it into triangles and arrange them like a jigsaw puzzle to cover the entire bottom and sides of the bowl. Cut a few slices from the other half into strips and use these strips to line the sides of the bowl, which only takes a few minutes and is very straightforward. Sprinkle the cake generously with your choice of liqueur; Kahlua is a great option.

In a medium pot (off the heat), whisk together the egg yolks and sugar briskly with a wooden spoon. Gradually add the flour, mixing well into the eggs and sugar. Heat the milk until it just comes to a boil, then add the vanilla. Slowly pour the hot milk into the egg mixture in small amounts, stirring well between additions to prevent curdling. Once fully combined, place the pot over medium heat and continue stirring continuously. When the mixture boils, cook for another 3 to 4 minutes until thickened, then remove from heat. Stir as it cools to prevent a skin from forming.

Melt the Chocolate: Melt the chocolate either in the microwave or in a heatproof bowl set over simmering water. Set aside to cool slightly.

In a medium bowl, whip 10 oz (300 ml) of heavy cream with instant coffee powder until stiff peaks form; set aside. To the cooled pastry cream, add 1 tablespoon Grand Marnier, 2 oz (55 g) grated chocolate, and fold in the whipped coffee cream gently until fully combined. Spread about two-thirds of this mixture evenly into the cake-lined bowl, filling it nearly to the top but leaving a hollow center; don’t worry if it isn’t perfect, it will come together in the end.

Fill the Zuccotto: Mix the melted chocolate into the remaining cream until the color is uniform, then spoon this chocolate cream into the hollow center of the zuccotto.

Use the remaining cake pieces to close the top of the zuccotto. Sprinkle with additional liqueur, cover tightly with plastic wrap, and refrigerate for at least 6 hours, preferably overnight for the best flavor and texture.

Whip the remaining 8 oz (235 ml) heavy cream with 2 teaspoons coffee powder and 2 tablespoons sugar until stiff peaks form. When ready to serve, remove the plastic wrap from the top, place a flat plate or cake stand over the bowl, and invert the zuccotto onto the plate. Remove the bowl gently—don’t worry if the cake has cracks, it will still taste amazing. Spread the coffee-flavored whipped cream evenly over the top and create swirls or peaks as you like. Finish by sprinkling with chocolate shavings, curls, or extra grated chocolate. Chill the decorated cake for an hour if possible, though it can be served immediately if needed.