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

In a large bowl, whisk together flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until even.

In a separate bowl, beat the eggs, vegetable oil, buttermilk, and vanilla until smooth.

Pour the wet mixture into the dry ingredients and stir gently until mostly combined.

Slowly add the hot water while stirring to loosen the batter; it will be thin but smooth.

Divide batter between the prepared pans and bake for 30 to 35 minutes, or until a toothpick comes out with a few moist crumbs.

Let cakes cool in pans for 10 minutes, then turn out to a rack to cool completely.

In a small saucepan over medium heat, combine blackberries and 1/4 cup sugar. Cook until berries break down and release juices, about 5 to 7 minutes.

Stir in the cornstarch slurry and lemon juice, simmer until thickened, then remove from heat and cool fully.

Whip the cream with powdered sugar and vanilla in a chilled bowl until stiff peaks form.

For extra stability, fold in 2 to 3 tablespoons of mascarpone or stabilized whipped cream.

Place one cake layer on your serving plate. Spread half the cooled blackberry compote over it.

Top with the second cake layer. Frost the top and sides with whipped cream.

Decorate with fresh blackberries, edible flowers, dark chocolate shavings, and a light dusting of cocoa powder.

Chill for at least 1 hour before slicing.