Adjust the oven racks to the upper-middle and lower-middle position and heat the oven to 350 degrees Fahrenheit. Grease 3 9-inch round cake pan, then dust with cocoa powder and line the bottoms with parchment paper.
Whisk the flour, baking soda, baking powder, and salt together in a large bowl.
In a medium bowl, whisk the boiling water, chocolate, and 1/2 cup of the cocoa powder together until smooth.
In another large bowl, beat the butter and sugar together with an electric mixer on medium speed until light and fluffy, 3-6 minutes. Beat in the eggs, one at a time, until combined, about 30 seconds. Beat in the sour cream and vanilla until incorporated.
Reduce the speed to low and beat in 1/3 of the flour mixture, followed by 1/2 of the chocolate mixture. Repeat with half of the remaining flour mixture and the remaining chocolate mixture. Beat in the remaining flour mixture until just incorporated.
Give the batter the final stir with a rubber scraper to make sure it is thoroughly combined. Scrape the batter into the prepared pans, smooth the tops, and gently tap the pans on the counter to settle the batter. Bake the cakes until a toothpick inserted in the center comes out with a few crumbs attached, 15-20 minutes, rotating and switching the pans halfway through baking.
Let the cakes cool in the pans for 10 minutes. Runs a small knife around the edge of the cakes, and then flip them out onto the wire racks. Peel off the parchment paper, and flip the cakes over again. Cool the cakes completely, about 2 hours.
PREP CHERRIES AND MAKE SYRUP:
Reserve 8 of the prettiest cherries for your garnish in a small bowl. Slice the remaining cherries in half and place in a medium bowl.
Simmer the reserved cherry water and sugar together in a medium saucepan over medium heat until the mixture is thickened, about 8 to 10 minutes.
Toss 1 tablespoon of the syrup with the cherries reserved for garnish. Toss 3 tablespoons more syrup with the halved cherries.
Poke the top of the cake layers thoroughly with the back of a wooden spoon and brush with the remaining syrup.
MAKE THE WHIPPED CREAM FROSTING:
Whisk the sugar and the cornstarch together in a small saucepan and slowly whisk in 1/2 cup of the cream. Bring the mixture to a simmer over medium heat, whisking constantly, until the mixture thickens, 2-3 minutes. Let the mixture cool to room temperature, about 30 minutes.
In a large bowl, whip the remaining 2 1/2 cups cream and the vanilla together with an electric mixer on low speed until frothy, about 30 seconds. Increase the mixer speed to medium and continue to whip the mixture until it begins to thicken, about 30 seconds. Slowly add the cooled cornstarch mixture and continue to whip until the mixture forms soft peaks, 1-3 minutes.
Line the edges of a cake platter with strips of parchment paper to keep the platter clean while you assemble the cake. Place one of the cake layers on the platter. Spread 1/2 cup of the whipped cream over the top, right to the edge of the cake.
Then, cover with half of the sliced cherries. Repeat with another cake layer, 1/2 cup more whipped cream, and the remaining sliced cherries.
Place the remaining cake layer on top and press lightly to adhere. Frost the cake with the remaining whipped cream.
Refrigerate the cake until it absorbs the soaking syrup, at least 2 hours or up to 24 hours.
Before serving, let the cake sit at room temperature, 30-60 minutes.
Make the chocolate shavings by placing a chunk of chocolate on a piece of parchment paper or wax paper. Use a paring knife to scrape the chocolate to form chocolate shavings.
Gently press the chocolate shavings into the sides of the cake.
Decorate the top of the cake with whipped cream, the reserved cherries, and chocolate shavings. Finally, slice and serve.
Because of the whipped cream in this cake, I do not recommend freezing it. It will be a soupy mess if you do.
This cake will cut into 10 hefty slices. You can easily get more slices out of the cake for smaller portions.