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

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

In a separate bowl, combine the room temperature eggs, vegetable oil, buttermilk, and vanilla extract. Whisk until uniform.

Pour the wet mixture into the bowl of dry ingredients. Stir gently until just combined, adding hot water gradually.

Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes or until a toothpick comes out clean. Cool in pans for about 10 minutes.

In a medium saucepan over medium heat, combine the blackberries with granulated sugar. Cook until juicy and soft.

Once bubbling, add the cornstarch slurry and lemon juice, stirring until thickened. Set aside to cool.

In a chilled mixing bowl, pour in the heavy whipping cream and powdered sugar. Whip until stiff peaks form.

Place one layer of the cooled chocolate cake on a serving plate. Spread an even layer of blackberry filling, allowing some to spill over the edges.

Place the second layer on top, then generously dollop whipped cream over the entire cake and garnish as desired.