Sift together the powdered sugar and cocoa powder to remove any clumps.
Place egg whites, powdered sugar and cocoa powder in a stand mixer bowl.
Attach the whisk and combine ingredients on low and then whip on high for 5 minutes.
Add in your room temperature butter in chunks. Whisk on high for 8-10 minutes until light and fluffy. Don't be afraid to let it whip, when it's done it will no longer taste like butter.
Optional: If your buttercream looks curdled, remove one cup of the buttercream and melt it in the microwave for 10-15 seconds until its JUST barely melted. Add the melted buttercream back into the whipping buttercream and it will make it all come together.
Add in your salt and vanilla extract. Mix until combined.
Optional: Remove the whisk and place the paddle attachment onto the mixer. Let your buttercream mix on low until the mixture is smooth and creamy and there are no bubbles left. This can take 15-20 minutes and works best with a double batch of buttercream and the entire paddle is covered.