Preheat the oven to 325 degrees F. Grease and line three 6 inch cake pans with parchment paper.

Soak cake strips in ice water.

In a medium bowl, whisk together the flour, sugar, cocoa powders, baking powder, and salt. Set aside.

In a large bowl, whisk together the milk, oil, eggs, and vanilla.

Add the dry ingredients to the wet and whisk until just combined.

Lastly, whisk in the hot coffee until fully combined. Set aside.

Divide the cake batter between the three prepared pans. Wrap the cake strips around the outside of the pans, then bake for 35-40 minutes or until a toothpick inserted comes out clean.

Turn the cakes out onto a wire rack to cool completely.

Once cool, use a serrated knife to cut the cakes in half to get 6 cake layers.

In a small saucepan, simmer the water and sugar over medium heat until sugar dissolves, or about 5 minutes.

Remove from heat, then stir in the coffee liqueur.

Pour the simple syrup into a jar and chill in the fridge until ready to use.

Brush the simple syrup onto the cooled cakes, then wrap them up in plastic wrap and freeze until ready to assemble.

For the chocolate espresso mousse

In a large bowl, combine the melted chocolate, espresso powder, salt, egg, egg yolk, and vanilla. Use an electric hand mixer to blend until smooth.

In a small saucepan, bring the water to a boil.

Add the boiling water to the chocolate mixture and mix until smooth.

In the same saucepan, bring 227g (1 cup) heavy cream to a simmer (190 F), then add to the mixture and blend until smooth.

In a separate medium bowl, add the remaining 170g (¾ cup) heavy cream and use an electric mixer to beat until stiff peaks form.

Add the whipped cream to the chocolate mixture and gently fold with a rubber spatula until fully combined.

Place in the fridge for at least one hour to set.

Wipe the bowl of a stand mixer and whisk attachment down with white vinegar.

Add the egg whites, sugar, and salt to the bowl and whisk until fully combined.

Place over a small saucepan of simmering water, and whisk frequently until it reaches 160 degrees F or until the sugar is completely dissolved and the mixture no longer feels grainy.

Move the bowl over to the stand mixer with the whisk attachment. Whisk on medium-high speed until medium peaks form.

Add the vanilla and continue whisking until stiff peaks form.

Turn the mixer down to low speed, and wait until the meringue cools down to at least 85 degrees F.

Cut the butter into cubes and add to the meringue a few pieces at a time, allowing them to fully incorporate before adding more.

Once all of the butter is added, it may look curdled, but keep beating for a few more minutes and it will come together.

Once the buttercream has mostly come together, change the whisk attachment to the paddle, add the espresso and cocoa powders, and keep mixing on low speed until homogenous and smooth.

Place the first cake layer on the cake stand. Using a 3 tablespoon cookie scoop, add three scoops of mousse and spread it into an even layer. Repeat with the remaining layers.

Spread a thin layer of buttercream (also called a crumb coat) all over the cake and chill in the fridge for 10-15 minutes.

Spread a thicker layer of buttercream along the sides and top of the cake, and use a cake scraper to get extra smooth sides.

Pipe the remaining buttercream in various patterns, as desired.

Leave the cake at room temperature for at least three hours to thaw before serving.