Place egg whites in a small bowl (make sure there is no trace of egg yolk). Cover with plastic wrap and poke a few large holes in it. Leave out at room temperature for 8 hours or in the refrigerator overnight. Bring to room temperature before using.

Place confectioners' sugar and fine almond flour into a fine mesh sieve over a bowl. Sift and let the bowl catch the sifted ingredients. Discard any large clumps in the sieve. Repeat twice and set aside dry ingredients.

Add half of the dry ingredients to the mixing bowl. Use a silicone spatula to gently fold the dry ingredients into the wet ingredients. Once mostly incorporated, add the remaining dry ingredients. Continue folding by going around the sides of the bowl with the spatula, then cutting through the center. The batter is done when it has a lava-like consistency. Do not over-mix. Pro tip: Try lifting the spatula out of the bowl and forming figure 8 with the batter without it breaking.

Transfer batter to a piping bag fitted with a round tip (about 1/2-inch in diameter). Pipe batter onto a silpat mat with macaron outlines or onto parchment paper. Pipe circles 1-inch wide and spaced 2 inches apart. Lift the baking sheet about 6 inches off the counter and drop it down. Repeat twice more, then rotate the pan 180° and bang the pans three more times (this releases air bubbles from the macarons). I also like to use a toothpick to pop any air bubbles I see at the top of the cookies.

Let the macarons sit at room temperature for 40 to 45 minutes and up to an hour. The macarons are ready to bake when dry to the touch (you should be able to "pet" them without leaving a mark).

Ten minutes before macarons are done drying, preheat the oven to 300° F. Bake macarons for 12 to 14 minutes, rotating the pan halfway through. Cool completely.

In the bowl of a stand mixer, beat the butter using the paddle attachment. Beat until creamy, about 2 minutes. With the mixer on low speed, gradually add in the confectioners' sugar until completely incorporated.

With the mixer on medium speed, add the caramel, vanilla and a pinch of salt. Mix until combined. If necessary, add milk for a thinner consistency.

Match up the cooled macarons according to size.

Transfer frosting to a bag with the desired tip. Pipe a border of buttercream on the bottom of one macaron shell. Place ½ teaspoon caramel sauce in the center and top with the matching shell. Repeat with remaining cookies.