Preheat oven to 375°F (190°C). Grease a 10x15-inch jelly roll pan. Line it with parchment paper, leaving a slight overhang on the short sides. Grease and flour (or dust with cocoa powder) the parchment paper. Lay a clean, lint-free kitchen towel on your counter and dust it generously with cocoa powder.

In a small saucepan, whisk together the reserved 2 tbsp of cherry liquid and the cornstarch until smooth. Stir in the drained cherries and 2 tbsp of sugar. Cook over medium heat, stirring constantly, until the mixture thickens and bubbles, about 3-5 minutes. Remove from heat and stir in the Kirsch, if using. Set aside to cool completely.

In a medium bowl, sift together the all-purpose flour, 1/3 cup cocoa powder, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the whisk attachment (or using a large bowl and a hand mixer), beat the eggs and 3/4 cup of sugar on high speed for 5-7 minutes. The mixture should be pale, thick, and triple in volume, falling from the whisk in a thick ribbon that holds its shape for a few seconds.

Reduce the mixer speed to low and beat in the vanilla extract. Sift the dry ingredients over the egg mixture in two batches, gently folding with a large spatula after each addition until just combined. Do not overmix. Gently fold in the melted and cooled butter until just incorporated.

Pour the batter into the prepared pan and spread it into an even layer with an offset spatula. Bake for 12-15 minutes, until the top of the cake springs back when lightly touched. Be careful not to overbake, as this can cause cracking.

Immediately run a knife around the edges of the pan to loosen the cake. Confidently invert the hot cake onto the prepared cocoa-dusted towel. Carefully peel off the parchment paper. While still hot, start from a short end and tightly roll up the cake with the towel inside. Place seam-side down on a wire rack to cool completely, about 1 hour.

While the cake cools, prepare the whipped cream. In a large, chilled bowl, beat the very cold heavy cream with an electric mixer on medium-high speed until soft peaks form. Add the sifted powdered sugar and vanilla, and continue to beat until stiff, pipeable peaks form.

Once the cake is completely cool, gently unroll it. Spread about two-thirds of the whipped cream evenly over the surface, leaving a 1-inch border. Dot the cooled cherry filling over the cream.

Carefully re-roll the cake tightly (without the towel). Place it seam-side down on a serving platter. Trim the ends with a serrated knife for a clean presentation.

Frost the outside of the roll with the remaining whipped cream or pipe rosettes along the top. Use a vegetable peeler to create chocolate shavings from the dark chocolate bar and sprinkle them generously over the cake. Garnish with cherries.

Refrigerate the cake roll for at least 1 hour before slicing and serving. This allows the filling to set and ensures clean slices.