Preheat your oven to 350°F (175°C) and prepare two 8-inch or 9-inch round cake pans by greasing them and lining with parchment paper.

In a large mixing bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, beat together eggs, vegetable oil, buttermilk, and vanilla extract until smooth.

Gradually pour the wet mixture into the dry ingredients, stirring gently, then add hot water and mix until smooth.

Divide the batter between the prepared pans and bake for 30–35 minutes, or until a toothpick comes out clean.

Cool the cakes in the pans for 10 minutes, then transfer to a wire rack to cool completely.

In a saucepan, combine blackberries and sugar over medium heat; cook until juicy. Stir in a cornstarch slurry and lemon juice until thick.

In a chilled bowl, whip heavy cream with powdered sugar until stiff peaks form.

Assemble the cake by layering one cake, blackberry filling, another layer, and topping with whipped cream.

Refrigerate the assembled cake for at least 1 hour before serving.