3 sticks (12 ounces) unsalted butter, cut into 1 1/2-inch chunks, at room temperature
Flavoring suggestions (see below)
Bring a few inches of water to a boil in a saucepan that can hold a heatproof stand mixer's bowl above the water.
Add the sugar, lemon juice, salt and egg whites to the mixing bowl, and whisk together by hand. Set the bowl above the boiling water and whisk until the mixture is warm to the touch and the sugar completely dissolves. Transfer to the stand mixer fitted with the whisk attachment and beat at medium-high speed until cool and the whites hold stiff peaks, 10 to 15 minutes.
Toss in a couple of chunks of butter at a time, making sure the pieces are incorporated before adding more. After all the butter is added, continue whisking on medium-high speed. The mixture will deflate and appear curdled. Continue whisking until the buttercream comes back together to form a smooth and spreadable consistency. Slowly drizzle in the optional flavoring while beating until smooth and light. (If the frosting is very soft or begins to break, refrigerate until set but still spreadable and then beat until light before using.)
Remove the buttercream from the stand mixer into a large bowl and, in three batches, fold in 6 ounces melted and cooled bittersweet chocolate.
Stir in the zest of half a lemon or orange.
Stir in 2 tablespoons coffee-flavored liqueur and 1 tablespoon vodka.