Sift your Cocoa Powder into a separate bowl and set aside.
Melt your chocolate over a double boiler and set aside.
Cut your butter into Tablespoon sized slices and set aside.
Mix together sugar and egg whites, using a whisk, in the bowl of a stand mixer and place over a pan with simmering water. Stir constantly until temperature reaches 160° F, or until all of the sugar is melted and mixture is foamy and white on the top. Remove from heat and put onto mixer with the whisk attachment. Mix on high.
Once meringue is room temperature, slow the mixer down to about 7, and start adding butter one tablespoon at a time. Do not add another slice until the first one disappears. After adding about 8-16 tablespoons, switch to the paddle attachment and continue adding butter until it is all incorporated. Then mix on high for 2 minutes.
With mixer set to stir, add your cocoa powder slowly followed by your powdered sugar. Scrape down the sides and bottom of your bowl and then add your vanilla and whip again for 2 minutes.
Set your mixer to stir once again and pour in your room temperature melted chocolate. Whip your buttercream again until fully incorporated. Scrape down the sides and bottom of your bowl and WHIP AGAIN!! If your buttercream seems too soft, stick the bowl in the fridge for 10-20 minutes and then re-whip.
To remove air bubbles, set mixer to stir and let it run for about 10 minutes. This will help release a lot of the excess air, leaving you with smooth, dreamy PERFECTLY PERFECT CHOCOLATE BUTTERCREAM!
***If your kitchen is particularly warm, wait to slice your butter until your meringue is whipping up.