Preheat your oven to 350°F (175°C) and grease and flour two 8-inch or 9-inch round cake pans.

In a large mixing bowl, whisk the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking powder, baking soda, and salt until combined.

In another bowl, beat the eggs, vegetable oil, buttermilk, and vanilla extract until mixed. Gradually add this wet mixture to the dry ingredients, stir until just incorporated, then add hot water and mix smooth.

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

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

In a saucepan, combine blackberries and a bit of sugar, cook until soft. Mix a cornstarch slurry with lemon juice, stir until thickened, and cool before using.

Whip heavy cream with powdered sugar until stiff peaks form. This will be your topping.

Layer one cake on a serving plate, spread blackberry filling over it, top with whipped cream, then place the second layer on top and frost the sides and top with remaining whipped cream.

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