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

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

In a separate bowl, beat the eggs gently, then add the vegetable oil, buttermilk, and vanilla extract. Whisk until well blended.

Gradually pour the wet mixture into the dry ingredients, stirring gently until just combined. Then, incorporate the hot water carefully.

Evenly divide the batter between the prepared pans and bake for 30 to 35 minutes.

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

In a saucepan over medium heat, combine the blackberries and sugar, stir until juicy and fragrant. Stir in cornstarch mixed with water and lemon juice.

In a medium bowl, whip the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.

Once cooled, place one cake layer on a serving platter, spread blackberry filling on top, then add the second layer and top with whipped cream.

Refrigerate the assembled cake for at least 1 hour before serving.