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

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

In a separate large bowl, beat the eggs with vegetable oil, buttermilk, and vanilla extract on medium speed until smooth.

Gradually incorporate the dry ingredients into the wet mixture, stirring gently. Slowly add the hot water while stirring.

Divide the batter evenly between the prepared cake pans and bake for 30-35 minutes, until a toothpick comes out clean.

Cool the cakes in their pans for 10 minutes before transferring to a wire rack to cool completely.

In a saucepan, combine blackberries and granulated sugar over medium heat, cooking until juices release. Stir in cornstarch slurry and lemon juice, cooking until thickened.

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

Assemble the cake by spreading blackberry filling between the layers, topping generously with whipped cream.

Refrigerate the assembled cake for at least one hour before slicing for best flavor and texture.