Prepare the pan and oven. Preheat the oven to 170 degrees Celsius. Line a 25 by 18 centimeter baking pan with parchment paper. Leave some overhang on the sides. This makes lifting the finished cake much easier.

Melt the chocolate and butter. Place the 180 grams of dark chocolate and the butter in a heatproof bowl. Set the bowl over a saucepan of gently simmering water. Do not let the bowl touch the water. Stir until the mixture is smooth and glossy. Remove from the heat and set aside to cool for a few minutes.

Make the brownie batter. In a large bowl, whisk the eggs and sugar with a hand whisk for about one to two minutes. The mixture should look well combined and slightly foamy, but not pale and thick. Slowly stir in the melted chocolate and butter mixture. Add the vanilla extract and salt.

Add the dry ingredients. Sift the flour and cocoa powder directly into the bowl. Add the 50 grams of chopped dark chocolate. Gently fold everything together with a spatula. Stop as soon as no streaks of flour remain. Overmixing will make the brownie tough.

Bake the brownie. Pour the batter into the prepared pan and spread it evenly with a spatula. Bake for 25 to 30 minutes. Start checking at the 25 minute mark. Insert a toothpick into the center. It should come out with a few moist crumbs attached, not wet batter. A completely clean toothpick means the brownie is overbaked.

Cool the brownie completely. Let the brownie cool in the pan on a wire rack. Do not rush this step. The cherry filling will slide off a warm brownie.

Make the sour cherry filling. Place the sour cherries, sugar, and lemon juice in a saucepan over medium heat. Cook for a few minutes until the cherries soften and release their juices. In a small bowl, mix the cornstarch with a little cold water until smooth. Pour this slurry into the cherries while stirring. Continue cooking for another one to two minutes. The mixture should thicken visibly. Remove from the heat and let it cool completely.

Spread the cherry filling. Once both the brownie and the cherry filling are fully cooled, spread the cherry layer evenly over the brownie base. Use an offset spatula or the back of a spoon. Try to keep the layers neat around the edges.

Make the white cream. In a chilled bowl, whip the cold heavy cream and powdered sugar until soft peaks form. Add the mascarpone. Continue beating on low speed just until the cream holds medium peaks. Do not overwhip, or the cream will become grainy.

Decorate the cake. Spoon the mascarpone cream over the cherry layer. Gently create soft waves on the surface with the back of a spoon. Scatter chocolate shavings over the top. Add a few fresh cherries if you like.

Refrigerate. Place the assembled cake in the refrigerator for several hours before slicing. This resting time allows the layers to firm up and the flavors to meld. Three to four hours is ideal. Overnight works even better.