Preheat your oven to 350°F (175°C). Grease and line the bottoms of three 8-inch round cake pans with parchment paper and lightly grease the parchment.

In the bowl of a stand mixer, sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt. Mix on low for 30 seconds.

In a separate bowl, whisk together the eggs, buttermilk, oil, and vanilla until smooth. Slowly pour the wet ingredients into the dry with the mixer on low. Mix until combined.

Carefully pour in the hot coffee. Mix on low until just combined, scraping the bowl as needed.

Divide the batter evenly among the prepared pans. Bake for 22-26 minutes or until a toothpick comes out clean.

Let the cakes cool in their pans for 15 minutes, then invert onto racks to cool completely.

Tip: Wrap cooled layers tightly in plastic wrap and freeze for 30 minutes for easier frosting.

In a medium saucepan, heat the milk over medium heat until steaming. Do not boil.

In a separate bowl, whisk the egg yolks, sugar, cornstarch, and salt until thick and pale yellow.

While whisking, drizzle half of the hot milk into the egg mixture to temper it. Pour the entire mixture back into the saucepan.

Cook over medium heat, whisking constantly, until thick, about 2-3 minutes.

Remove from heat and whisk in chocolate, butter, rum, and vanilla until smooth.

Press plastic wrap on the surface and refrigerate for at least 2 hours, or until cold.

In the bowl of a stand mixer, beat softened butter until smooth, about 3 minutes.

Gradually add sifted confectioners’ sugar and cocoa powder, mixing until incorporated.

Add cooled coffee, vanilla, and salt and beat on medium-high for 3-4 minutes until fluffy.

Adjust consistency with milk or sugar as needed.

Level the domed tops of the cake layers with a serrated knife.

Place one layer on a serving plate, spread half of the chocolate-rum filling on top.

Place the second layer on top and spread with the remaining filling.

Top with the last layer and apply a thin crumb coat of mocha buttercream over the cake.

Refrigerate for 20 minutes to set, then frost with remaining buttercream.

Garnish with chocolate shavings and espresso beans. Let sit for 30 minutes before slicing.