Line 2 large baking sheets with parchment paper or silicone macaron mats. If using parchment, you can trace 1½-inch circles on the underside as a piping guide. Make sure your mixing bowl and whisk are completely clean, dry, and grease-free; wipe with a bit of vinegar or lemon juice if desired.

In a medium bowl, combine the almond flour and powdered sugar. Sift together 2–3 times, pressing any small clumps through the sieve. Discard any large grainy bits of almond that remain.

Place the room-temperature egg whites and salt in the bowl of a stand mixer fitted with the whisk attachment (or use a hand mixer). Begin whipping on medium speed until foamy, then slowly sprinkle in the granulated sugar, about 1 tablespoon at a time. Once all the sugar is incorporated, increase to medium-high and whip until stiff, glossy peaks form. Add the vanilla extract and gel food coloring (if using) and mix just to combine.

Add half of the almond flour–powdered sugar mixture to the meringue. Using a rubber spatula, fold gently, scraping around the bowl and through the center. Add the remaining dry mixture and continue folding in a slow, deliberate motion, pressing the batter against the side of the bowl and then scooping and turning. Stop when the batter reaches a lava-like consistency and flows off the spatula in a ribbon that folds over itself and disappears back into the bowl in about 10–20 seconds.

Transfer the batter to a piping bag fitted with a round tip (about 1/2 inch opening). Pipe 1½-inch rounds onto the prepared trays, holding the bag straight up and close to the tray. Stop squeezing, then give a small twist of the wrist to finish each shell. Try to keep the shells similar in size.

Firmly tap each tray on the counter 5–6 times to release air bubbles. If you see any bubbles on the surface, pop them gently with a toothpick. Let the shells sit at room temperature for 30–60 minutes, or until the tops look dull and feel dry to the touch.

While the shells rest, preheat the oven to 300°F (150°C). Bake one tray at a time on the center rack for 14–17 minutes, rotating the tray halfway through if your oven has hot spots. The shells are done when they have risen with nice feet, the tops don’t jiggle when gently nudged, and they peel off the parchment cleanly once cooled. Cool completely on the trays.

For any buttercream filling, beat the softened butter with an electric mixer until creamy. Add powdered sugar, a pinch of salt, and flavorings (vanilla extract for vanilla buttercream; raspberry jam for raspberry buttercream; or the dissolved espresso mixture for coffee buttercream). Beat until light and fluffy, adding heavy cream or milk 1 teaspoon at a time until you reach a smooth, pipeable consistency.

Place the chopped chocolate in a heat-safe bowl. Heat the heavy cream until steaming but not boiling, then pour it over the chocolate. Let sit for 1–2 minutes, then stir gently until smooth. Stir in the butter if using. Let cool until thickened but still spreadable, stirring occasionally as it cools.

Carefully remove the cooled macaron shells from the parchment. Pair them by size. Pipe or spoon a small dollop of your chosen filling onto the flat side of one shell (about 1 teaspoon for standard-size macarons), then gently press a second shell on top, twisting slightly so the filling just reaches the edges. Avoid overfilling.

Place the filled macarons in a single layer in an airtight container. Refrigerate for at least 24 hours (up to 48 hours) to allow the shells to absorb some moisture from the filling and develop a chewy interior. Before serving, let the macarons sit at room temperature for 20–30 minutes.