Preheat your oven to 350°F (177°C). Grease and dust two 8-inch round cake pans with cocoa powder. In a stand mixer, combine the dry ingredients: flour, sugar, cocoa powder, baking soda, baking powder, and salt. In a separate bowl, whisk together the wet ingredients: vegetable oil, buttermilk, hot water, eggs, and vanilla extract until smooth. Gradually mix wet into dry until combined.

Divide the cake batter evenly between the two prepared pans. Bake for about 45 minutes, or until a toothpick comes out mostly clean. Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

In a mixing bowl, beat softened butter and caster sugar together for about 3 minutes until light and fluffy. Stir in melted chocolate and vanilla extract. Add eggs one at a time, whipping for 5 minutes after each addition.

Chill the frosting for about 30 minutes. Rewhip every 10 minutes to maintain texture.

Place one cooled layer on a platter, pipe frosting between layers, and cover the top and sides with remaining frosting.

Sprinkle chocolate flakes or sprinkles on top if desired. Chill in the refrigerator for at least one hour before serving.