Preheat Oven: Set your oven to 350°F (175°C) and prepare two 8-inch or 9-inch cake pans by greasing and lining them with parchment paper.

Mix Dry Ingredients: In a large bowl, whisk together 2 cups of all-purpose flour, 1 ½ cups of granulated sugar, ¾ cup of cocoa powder, 1 ½ teaspoons of baking powder, 1 ½ teaspoons of baking soda, and 1 teaspoon of salt until well combined.

Combine Wet Ingredients: In a separate bowl, beat together 2 large eggs, 1 cup of vegetable oil, 1 cup of buttermilk, and 2 teaspoons of vanilla extract until smooth.

Combine Mixtures: Gently add the wet ingredients to the dry ingredients, mixing until just combined. Stir in 1 cup of hot water to create a thin batter.

Bake Cakes: Divide the batter evenly between prepared cake pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Cooling: Once baked, allow the cakes to cool in their pans for about 10 minutes before transferring them to a wire rack to cool completely.

Prepare Blackberry Filling: In a saucepan, combine 2 cups of blackberries with ½ cup of granulated sugar, cooking until the berries soften. Add 2 tablespoons of cornstarch slurry and juice from 1 lemon, cooking until thickened. Allow to cool.

Whip Cream: Using a mixer, beat 1 cup of heavy whipping cream with ¼ cup of powdered sugar and 1 teaspoon of vanilla extract until stiff peaks form.

Assemble Cake: Place one layer of the cooled cake on a serving plate, spread a generous amount of blackberry filling, and add the second cake layer. Top with whipped cream and garnish.

Chill and Serve: Chill the assembled cake in the refrigerator for at least 1 hour before serving.