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

In a large mixing bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk for about 1 minute.

In a separate bowl, beat together eggs, vegetable oil, buttermilk, and vanilla extract until well blended, approximately 2 minutes.

Gradually pour the wet ingredients into the dry ingredients, stirring gently until just mixed.

Slowly mix in hot water into the batter, stirring until smooth, about 1 minute.

Divide the batter evenly between the prepared cake pans. Bake for 30 to 35 minutes, or until a toothpick inserted comes out clean.

Once baked, let them cool for 10 minutes, then invert onto wire racks to cool completely.

In a medium saucepan over medium heat, combine blackberries and sugar. Stir until the blackberries soften, about 5 minutes.

Add cornstarch mixed with water and lemon juice, and cook until thickened, approximately 3–5 minutes. Allow cooling.

In a mixing bowl, whip heavy cream with powdered sugar and vanilla extract until stiff peaks form, about 3–5 minutes.

Assemble the cake by placing one layer on a serving plate, spreading blackberry filling on top, and then adding the second layer. Top with whipped cream.

Cover and refrigerate for at least 1 hour before serving. Garnish if desired.