Bake the Cake:

1. Preheat the oven to 350°F. Butter two 9-inch round pans, line the bottoms with parchment paper, and butter the parchment. Dust with flour and tap out the excess.
2. In a large bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
3. Add the milk, eggs, vegetable oil, and vanilla. Mix until smooth. Slowly add the boiling water and mix until fully combined—the batter will be thin, and that’s perfect.
4. Divide the batter evenly between the prepared pans. Bake for 35 minutes, or until a toothpick inserted in the center comes out clean.
5. Let the cakes cool in the pans on a wire rack for 15 minutes. Carefully invert the cakes onto the rack, peel off the parchment, and let them cool completely.

Make the Chocolate Buttercream:

1. Melt the bittersweet and semisweet chocolate in a heatproof bowl set over a pot of simmering water. Let cool to room temperature.
2. In a stand mixer fitted with the whisk attachment, whip the egg whites, sugar, cream of tartar, and salt over simmering water until the mixture reaches 160°F and the sugar dissolves. Transfer to the mixer and beat on high speed until cool and fluffy, about 8 minutes.
3. With the mixer on medium speed, add the butter one tablespoon at a time until incorporated. Beat in the vanilla, espresso mixture, and rum (if using). Add the melted chocolate and mix until smooth and glossy.

Assemble the Cake:

1. Place one cake layer on a serving plate or cake stand. Spread 1 cup of buttercream evenly over the top.
2. Add the second cake layer and frost the top and sides generously. Use an offset spatula for a smooth finish—or embrace the rustic look.
3. If you’re feeling fancy, reserve some frosting to pipe decorations on the top! I won’t lie to you, I couldn’t pipe a design on a cake if I had a gun pointed between my eyes!