Preheat your oven to 300°F (150°C). Line two baking sheets with parchment paper or silicone baking mats.

In a mixing bowl, sift together the almond flour and powdered sugar. Set aside.

In a clean mixing bowl, whip the egg whites with a pinch of salt using a stand mixer or hand mixer until foamy. Gradually add the granulated sugar and continue to whip until stiff peaks form.

Gently fold the sifted almond flour and powdered sugar into the meringue in three parts. If using food coloring, add it during this step.

Transfer the batter to a piping bag fitted with a round tip. Pipe small circles onto the prepared baking sheets, leaving space between each macaron. Tap the baking sheets on the counter to remove air bubbles.

Let the piped macarons sit at room temperature for 30-60 minutes, or until they form a skin and are no longer sticky to the touch.

Bake in the preheated oven for 15-20 minutes, or until the macarons have risen and formed feet. Allow to cool completely on the baking sheets.

In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and add the chopped dark chocolate and butter. Stir until smooth and glossy.

Once the macaron shells have cooled completely, pair them up by size. Pipe a small amount of the dark chocolate filling onto the flat side of one shell, then gently press another shell on top.

Place the assembled macarons in an airtight container and refrigerate for at least 24 hours to allow the flavors to meld. Let them come to room temperature before serving.