Preheat oven to 350°F/175°C. Line one 6" round pan with a parchment round and grease with non-stick baking spray.

Mix together 1/3 cup flour, 1/2 cup sugar, 2 Tbsp cocoa powder and 1/2 tsp baking powder with a whisk until combined.

Mix 1/4 cup melted and cooled butter and 1 egg white into the dry ingredients and whisk until combined. The batter will be thick.

Add 1/4 cup buttermilk and 1/2 tsp vanilla extract. Mix until combined, and scrape the sides and bottom of the bowl with a rubber spatula to make sure everything is properly mixed together.

Pour the batter into the prepared cake pan and bake for 25-30 minutes or until a toothpick comes out with a few moist crumbs.

Allow the pan to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan. Flip the pan onto a wire rack and the cake layer cool fully.

Use a serrated knife to level the top of the layers, then cut out 4 small rounds of cake using a 2 1/2 inch circle cutter. The leftover bits of cake can be used to make cake pops or cake balls.

While the cake layer bakes and cools, make the chocolate buttercream frosting.

Beat 1/2 cup butter on a medium high speed for 30 seconds with a whisk attachment or hand mixer until smooth and lighter in color.

Mix in 1 tsp vanilla extract and 1/4 cup baking cocoa on a low speed.

Slowly mix in 1 1/2 cups of powdered sugar on a low speed.

Once you've added in all the powdered sugar, add 1 Tbsp heavy cream or milk to make the frosting easier to mix.

Continue to mix on low speed for a couple minutes until the desired consistency is reached. 

If the frosting is too thick, add in additional cream (1 tsp at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).

Place the frosting in a small piping bag and set aside.

Stack and frost the mini chocolate cake layers on a small dessert board or plate using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of chocolate buttercream between the cake layers with a small offset spatula.

Spread a thin coat of frosting around the cake, fully cover the cake layers.

Smooth using a bench scraper or acetate sheet then chill the cake in the fridge (10 minutes) or freezer (5 minutes) until the frosting is firm to the touch.

Add a second, thicker layer of frosting to the cake, then decorate as desired! I melted some dark chocolate to add drips around the chilled cake and added some chocolate sprinkles around the base.