For the cake: Preheat the oven to 350 degrees F. Line a quarter-sheet pan with parchment paper overlapping the longer sides, then spray with baking spray, ensuring the exposed sides of the pan are well-greased.
Combine the flour, granulated sugar and salt in a large bowl. Stir together and set aside.
Mix the buttermilk, baking soda, vanilla and egg in a small pitcher. Set aside.
Melt the butter in a small saucepan, then add the cocoa. Whisk together to combine. Add 1/2 cup water and bring to a boil, then cook for 10 to 15 seconds. Add the chocolate mixture to the flour and stir together for a moment to cool slightly. Add the egg mixture and stir until smooth. Add the batter to the prepared pan and bake until cooked through in the center, about 15 minutes. Set aside and allow to cool completely, about 30 minutes.
For the cherry sauce: Combine the cherries, granulated sugar and liquor in a small saucepan over medium-high heat. Bring to boil and cook, stirring occasionally, until the liquid has reduced and thickened, 6 to 7 minutes,. Remove from the heat and set aside to cool to room temperature.
For the whipped cream: Whip together the heavy cream, powdered sugar and vanilla with a stand mixer until stiff peaks form, 3 to 4 minutes. Cover and transfer to the refrigerator until ready to build the cake.
Once the cake has cooled completely, flip it onto a cutting board and remove the parchment paper. Cut the cake in half, then cut each piece in half again, giving you 4 equal pieces.
Carefully put the first piece of the cake on a cake stand or platter. Spread a quarter of the whipped cream on top, then scatter a third of the cherry mixture over the top. Repeat two more times, trying to build the cake layers as straight and even as possible. Place the final piece of cake on the top and spread the remaining whipped cream over the top. Grate the chocolate bar over the top of the whipped cream for the finishing touch.
Chill the cake for 20 minutes to help firm up the whipped cream before slicing and serving.