Preheat oven to 350°F (175°C). Grease and line three 8-inch round cake pans with parchment paper.
Whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt in a large bowl.
Add buttermilk, oil, eggs, and vanilla. Stir until smooth. Gradually mix in hot water to form a thin, glossy batter.
Divide batter evenly among pans and bake for 25-30 minutes. Cool in pans for 10 minutes, then transfer to wire racks.
Heat heavy cream in a saucepan until it simmers. Pour over chocolate, let sit 2 minutes, then stir until smooth. Cool slightly.
Spread cherry pie filling on one cake layer, drizzle with ganache. Repeat with layers, then frost the entire cake with ganache. Garnish with cherries and remaining filling.