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

In a large bowl, sift or whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, whisk eggs, vegetable oil, buttermilk, and 2 teaspoons of vanilla until smooth.

Pour the wet ingredients into the dry ingredients and stir just until combined. Add the hot water and stir until smooth.

Divide batter evenly between prepared pans. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Let cakes cool in pans for 10 minutes, then invert onto wire racks to cool completely.

In a small saucepan, combine blackberries and 1/4 cup sugar. Heat over medium, stirring until berries break down and become juicy (about 5–8 minutes).

Stir in cornstarch slurry and bring to a simmer until thickened (about 1–2 minutes). Remove from heat, stir in lemon juice, and cool fully.

Chill the mixing bowl and whisk for 10 minutes. Beat heavy cream with powdered sugar and vanilla to stiff peaks.

Place one cake layer on a serving plate. Spread cooled blackberry filling evenly over it. Top with the second cake layer.

Frost the top and sides with whipped cream, then decorate with fresh blackberries, edible flowers, and chocolate shavings.

Chill assembled cake for at least 1 hour before slicing.