Sift together almond flour and powdered sugar twice. Set aside.
In a clean medium bowl, beat the egg whites on medium low speed until foamy. Add cream of tartar and salt. Then slowly add sugar one tablespoon at a time. Continue to beat the egg whites on medium low speed until hard peaks form. (TIP: Stir in gel or powder food coloring, if desired, when the meringue reaches soft peak stage.)
Sift almond flour mixture into the meringue, and gently fold it with a spatula until batter is just combined. TIP: Making french macarons is all about the technique. This is the most crucial step of all. Let me try to describe the folding motion as best as I can. Run the spatula clockwise from the bottom, up around the sides and cut the batter in half. I suggest to count every fold, I do about 50 folds, and then I start testing the batter on a small plate after every couple folds. To test, drop a small amount of batter on a plate. If the small peaks dissolve into the batter on its own in about 10 seconds, the batter is ready. If not, fold couple more times and test again. Be careful not to over-fold the batter.
Transfer the batter into a pastry bag with a round tip. Pipe about 1.5-inch rounds about an inch apart on prepared baking sheets.
Tap the baking sheets firmly on the counter a few times to get rid of any air bubbles. You can also use a toothpick to burst a some large air bubbles.
Let the macarons sit out on the counter for at least 15-30 minutes, maybe up to couple hours, depending on humidity. When you lightly touch the macarons, and the batter does not stick to your finger, then it’s ready to go into the oven. While waiting, preheat the oven to 300°F (150°C).
Bake the macarons one sheet at a time for about 18-20 minutes. It’ll take longer for larger macarons. To test for doneness, touch a macaron lightly and try to move it. If it slides and wobbles, bake a minute or so longer. The cooked macarons should be firm to touch and the base shouldn’t move.
Cool the macarons on the sheet for 10-15 minutes, and then transfer the macarons to a wire rack to cool completely.
To make the buttercream filling, in a medium bowl, beat the egg yolks slightly with a whisk and add sugar . Continue to whisk until the mixture is pale and sugar is mostly dissolved. Stir in the milk. Transfer the egg yolk mixture into a small saucepan and heat over low heat, stirring frequently. Continue to cook until it’s thick like pudding, about 5 minutes. Transfer the mixture back to the bowl and bring it to room temperature. Stir in the butter in three batches. Add vanilla extract and continue to mix until smooth.
To assemble, transfer the buttercream filling into a pastry bag and fill the macarons.
Also, you can also add or subtract items as per your requirement.