Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans and line with parchment paper.

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

In another bowl, whisk together eggs, oil, buttermilk, and vanilla until smooth.

Gradually combine wet and dry ingredients, stirring until just combined. Stir in hot water.

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

Let cake layers cool in pans for 10 minutes, then invert onto a wire rack to cool completely.

In a saucepan, cook blackberries and sugar over medium heat until juicy, then add lemon juice and cornstarch mixture to thicken.

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

Assemble the cake by placing one layer on a platter, spreading blackberry filling, topping with the second layer, and frosting with whipped cream.

Chill the assembled cake in the refrigerator for at least one hour before serving.