Preheat your oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment paper.

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

In another bowl, beat together the wet ingredients: eggs, vegetable oil, buttermilk, and vanilla extract.

Gradually add the wet mixture to the dry mixture, stirring until combined, then mix in hot water.

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

Allow the cakes to cool in pans for 10 minutes, then transfer to wire racks to cool completely.

In a saucepan, heat blackberries with sugar, cook until juices release, and mix in cornstarch and lemon juice. Set aside to cool.

Whip the cream with powdered sugar until stiff peaks form.

Assemble the cake by layering and spreading berry filling between the layers, then top with whipped cream.

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