Preheat your oven to 350°F (175°C). Grease and line two 8- or 9-inch round cake pans with parchment paper.

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

In a separate bowl, beat the eggs, vegetable oil, buttermilk, and vanilla extract. Gradually add to the dry ingredients and stir to combine. Fold in the hot water.

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

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

In a saucepan, combine blackberries and some sugar. Cook for 5–7 minutes, stir in cornstarch slurry and lemon juice, and cook until thickened.

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

Once cooled, place one cake layer on a platter, spread blackberry filling over it, and top with the second layer. Spread whipped cream topping over the whole cake.

Garnish with fresh blackberries, chocolate shavings, or edible flowers before serving.