Preheat the oven to 150°C (300°F). Line two baking sheets with parchment paper.
In a bowl, sift together almond flour and powdered sugar to remove any lumps.
In a separate bowl, whip the egg whites until foamy. Gradually add granulated sugar and cream of tartar, continuing to whip until stiff peaks form.
Gently fold the almond flour mix into the meringue, being careful not to deflate the mixture.
Transfer the batter to a piping bag fitted with a round tip. Pipe uniform rounds on the prepared baking sheets.
Let the piped macarons sit at room temperature for 30-60 minutes until a skin forms on the surface.
Bake the macarons in the preheated oven for 15-20 minutes, rotating halfway through.
Let the macarons cool completely before removing them from the parchment.
To make the ganache, heat the heavy cream in a saucepan until just boiling. Remove from heat and stir in the chopped dark chocolate and butter until smooth.
Chill the ganache until it thickens slightly, then transfer to a piping bag.
Match the cooled macaron shells into pairs of similar size. Pipe a dollop of ganache onto one shell and sandwich with its pair.
Refrigerate for at least 24 hours to allow the flavors to meld. Serve them at room temperature.