Step 1Push the almond meal or flour, along with the powdered sugar and cocoa, through a strainer into a large bowl and whisk to blend.
Step 2Beat the egg whites in a mixer with a whisk attachment over low to medium speed until foamy, then increase the speed and continue just until they hold glossy, medium-firm peaks. Gently fold in the dry ingredients in 3 or 4 additions. When the dry ingredients are all incorporated, the mixture will be runny and look like cake batter.
Step 3Spoon half the batter into a pastry bag fitted with a half-inch round tip and, keeping the bag vertical and 1 to 2 inches above the sheet, pipe rounds about 1 1/2 inches in diameter and 2 inches apart onto a parchment-lined baking sheet.
Step 4Rap the pan against the counter (or don't), refill the bag and pipe macarons onto the second sheet. Set the rounds aside in a cool, dry place for 30 minutes to rest.
Step 5Meanwhile, center a rack in the oven and heat it to 425 degrees.
Step 6Work with one baking sheet at a time. Dust the tops of the macarons lightly with cocoa and put the baking sheet on top of a spare sheet. Slide the set-up into the oven and immediately reduce the temperature to 350 degrees. Prop the door open slightly using a wooden spoon (to reduce the heat as the macarons continue to rise and dry). The heat of every oven will vary; if the oven cools too quickly, do not prop open the door and instead quickly open and close the oven door every few minutes to gently release excess heat.
Step 7Bake for 10 to 12 minutes, or until the macarons are smooth and just firm to the touch. Transfer the sheet to a cooling rack.
Step 8Bring the oven temperature back to 425 degrees and repeat with the second sheet of macarons.
Step 9As soon as the oven has been reset, remove the macarons from the parchment . Lift the paper at one corner and pour a little hot water onto the baking sheet underneath the paper. Tilt the sheet to evenly dampen the parchment and leave the macarons on the paper for 15 seconds. Peel them off the paper and place them on a cooling rack. Match them up for sandwiching.
Step 10Pipe a generous amount of filling (about 1 teaspoon, or as desired) on one half of the macs and top with their mates. Pack the sandwiched cookies in a container and refrigerate for 24 hours (or for up to 4 days) before serving. Remove from the refrigerator 30 minutes before serving.