Prepare Cake Layers:
Preheat oven to 350°F (175°C). Grease and dust two 9-inch pans with cocoa powder.
Whisk flour, sugar, cocoa, baking powder, baking soda, and salt. Create a well.
In another bowl, whisk eggs, buttermilk, oil, vanilla. Combine with dry ingredients until just mixed.
Slowly add boiling water, stirring until smooth and thin.
Divide batter evenly; bake 30–35 minutes. Test with toothpick. Cool completely.
Make Cherry Filling:
Combine cherries, sugar, cornstarch, and lemon juice in a saucepan.
Cook over medium heat, stirring until thick and glossy (~8-10 mins). Cool before use.
Prepare Frosting:
Beat butter until fluffy. Gradually add powdered sugar and cocoa, alternating.
Add heavy cream and vanilla; beat until smooth and spreadable.
Assemble Cake:
Place one cake layer on plate; spread half cherry filling. Add second layer.
Spread remaining cherry filling, then top with frosting on sides and top.