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Preheat the oven to 350°F.
Brush melted butter on two 6” round cake pans. Line the bottom of the cake pan with a piece of round parchment paper and brush the parchment paper with butter as well. Set aside.
Add buttermilk, oil, melted butter, eggs, and vanilla extract to a large bowl. Beat with a whisk until combined.
Sift together flour, cocoa powder, baking soda, and baking powder into a medium mixing bowl. Add sugar and salt to the flour mixture, and mix thoroughly with a whisk. Add the flour mixture to the liquid mixture, and whisk until a smooth batter forms.
Divide batter evenly among the two prepared cake pans and bake for 30 - 32 minutes until a toothpick inserted in the middle of the cake comes out clean with just a few crumbs attached. Use a kitchen scale to divide the cake batter if you want to be precise, approximately 390 g of batter per pan. This will also help the cake bake evenly.
Remove from the oven and let cool on a wire rack for 10 - 15 minutes. When you can touch the cake pans, use a butter knife to loosen the sides, flip the pan over to remove the cake from the pan. Peel off the parchment paper, and allow the cakes to cool completely on a wire rack before adding frosting.
Melt the chocolate in a heat proof bowl over a pot of simmering water just until most is melted. Continue to stir as you take it off the heat until all the chocolate is completely melted and smooth. Set aside to cool.
Add egg white and sugar to another heat proof bowl and set it over the pot of simmering water, stirring constantly until all the sugar dissolves, the egg white is frothy and the mixture reaches around 160°F. This will take around 7 - 10 minutes.
Transfer the warm egg mixture to a stand mixer bowl and beat with the balloon whisk attachment on medium low speed until stiff peak forms, stream in the vinegar or lemon juice while beating.
Once the meringue peak is stiff, shiny and smooth, start adding the room temperature cubed butter while stirring, one cube at a time and allow it to incorporate into the meringue.
After all the butter is added, stream in the vanilla extract, increase the mixer to high speed and beat until the buttercream comes together and smooth. This will not take very long, only a few minutes.
Stop the mixer and add the completely cooled melted chocolate and cocoa powder, stir to incorporate, and then beat on high speed until the buttercream is fluffy.
When the cakes are completely cool, place 1 layer on a cake stand or plate. Put a few pieces of parchment paper underneath the cake to cover the cake stand or plate. This is to keep the frosting from smearing everywhere.
Add about ½ cup of frosting on top of the first layer and even it out with an offset spatula. Add the second layer of cake on top.
Frost the side and top with the rest of the frosting or just enough to your liking. If there is any leftover frosting, add piping decoration, or simply add sprinkles. Carefully pull out the pieces of parchment paper underneath the cake.
Refrigerate for at least 30 minutes or until ready to serve.