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

In one bowl, whisk together the dry ingredients: all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, mix the wet ingredients: eggs, vegetable oil, buttermilk, vanilla extract, and hot water.

Combine the dry and wet ingredients, stirring until smooth. Divide the batter evenly into the prepared pans and bake for 30 to 35 minutes.

Allow the cakes to cool completely in their pans.

Place the blackberries and the 1/4 cup of granulated sugar into a saucepan over medium heat. Cook until the blackberries are juicy.

Mix together the cornstarch and water to form a slurry. Add this to the saucepan along with the lemon juice, stirring until the mixture thickens.

Remove from heat and set aside to cool completely.

In a large mixing bowl, beat the heavy whipping cream with the powdered sugar and vanilla extract until stiff peaks form.

Once the cakes are cool, place one layer on a serving platter. Spread half of the blackberry filling over this layer, then top with a generous portion of whipped cream.

Place the second cake layer on top and repeat the process with the remaining blackberry filling and whipped cream.

For the best taste, chill the assembled cake for at least one hour before slicing.