Preheat your oven to 350°F (175°C). Prepare a 6-inch round cake pan by lining the bottom with parchment paper and greasing the sides.

In a medium mixing bowl, combine the all-purpose flour, granulated sugar, sifted cocoa powder, baking powder, and a pinch of salt. Whisk until fully incorporated.

Add the melted salted butter and room temperature egg white. Mix until well combined, then pour in the buttermilk and vanilla extract, stirring until smooth.

Pour the batter into the prepared pan and bake for 25-30 minutes until a toothpick inserted comes out clean.

Let the cake cool in the pan for 10 minutes before inverting onto a wire rack. Once cool, level the top and slice into four rounds.

In a large bowl, beat the softened salted butter until creamy. Gradually mix in the sifted cocoa powder and powdered sugar, alternating with heavy cream until smooth.

Assemble by layering the cake rounds with chocolate buttercream between each. Cover the exterior with more frosting.