Sift together the almond flour, icing sugar, and cocoa powder in a bowl.
In a clean, dry mixing bowl, beat the egg whites until they start to foam. Gradually add the granulated sugar while continuing to beat. Keep whipping until you reach stiff peaks. The mixture should be glossy and hold its shape.
Gently fold the sifted dry ingredients into the meringue. Be careful not to overmix; you want a smooth, thick batter. This is called the macaronage process.
Transfer the batter into a piping bag fitted with a round tip. Pipe small rounds (about 2 inches in diameter) onto the prepared baking sheets. Leave some space between each macaron.
Let the piped macarons sit at room temperature for about 45 minutes to form a skin. You will know they are ready when you can lightly press on the top of the shell and it is no longer sticky. In the meantime, preheat your oven to 300°F (150°C).
While the macarons are resting, heat the cream until it's almost boiling. You can do this on the stove or in the microwave. Pour it over the dark chocolate and let it sit for a minute. Stir until smooth, creating a ganache. Add the salt, and stir. Set this aside to thicken, until it's about the same texture as peanut butter.
Once the macarons have developed a skin, bake them in the preheated oven for about 15-18 minutes. Let them cool completely on the baking sheet before trying to remove them.
Once the macarons have cooled completely, match them up by size. Pipe a ring of chocolate ganache around the border of one shell, then pipe a small dollop of marshmallow fluff into the centre. Sandwich with another macaron shell.
Melt the 1/4 cup of dark chocolate and drizzle it over the macarons. Add miniature marshmallows on top for extra hot cocoa flair.
Allow the macarons to set in the refrigerator for at least 12 hours. This helps the flavours meld and greatly improves the texture.