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

Add the egg whites and sugar 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 (70 degrees C) or until the sugar is completely dissolved and the egg whites do not feel grainy.

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

Add the salt & 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 (30 degrees C).*

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 will 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 and keep mixing on low speed until homogenous and smooth.

Weigh out and divide the buttercream up into three parts, leaving one part in the bowl of a stand mixer.

In one bowl, add the sifted cocoa powder and mix with a rubber spatula until fully combined.

In the bowl of a stand mixer fitted with a paddle attachment, add the espresso powder and mix on low speed until fully combined and the granules have dissolved.

In a large piping bag fitted with your desired piping tip, use a rubber spatula to spread the vanilla buttercream along one portion of the inside, then repeat with the chocolate and espresso flavors.

Pipe onto cakes, cupcakes, or macarons.