To make the batter: In the bowl of a stand mixer fitted with the whisk attachment, add the egg whites, cane sugar, and vanilla extract. Whisk, starting on low to avoid splattering, and slowly increasing the speed to high, until stiff peaks form.
Sift the powdered sugar, almond flour, and cocoa together into the bowl of meringue. Turn the mixer on low for 10 to 15 rotations until the dry ingredients start to blend into the meringue. Pulse the mixer on medium-high 4 to 5 times to incorporate the batter.
Remove the mixing bowl from the mixer, and gently stir the batter with a spatula just until there are no pockets of egg whites or dry bits of almond flour. Don’t overstir. Transfer the batter to a piping bag fitted with Ateco tip #804.
Pipe the macaron batter onto the prepared cookie sheets (30 should fit on each cookie sheet). Holding a cookie sheet firmly in both hands, and making sure the parchment paper doesn’t slide around, bang the cookie sheet on the counter a few times. Rotate the cookie sheet and bang it a few more times. Repeat with the other cookie sheet. This will smooth out the batter and will help form the macarons’ famous “feet.”
Let rest for 45 minutes, or until the rounds of batter have formed shells.
Preheat the oven to 350°F.
Bake for 10 minutes or until set and dry. Let cool completely on the cookie sheets.
To make the filling: In the bowl of a stand mixer fitted with the paddle attachment, add the powdered sugar, butter, cocoa, and milk. Mix on low until combined, increase the speed to high, and mix for 1 minute more, or until light and fluffy. Transfer the filling to a piping bag with Ateco tip #864.
Pair the macarons together by size, and turn over every other one. Pipe a swirl of filling into the center of each turned over macaron. Sandwich the pairs together.
Serve immediately.
Notes
High Altitude — Bake at 350°F for 8 minutes or until the macs look dry and set.