Prepare two baking sheets with silicone mats and fit a large piping bag with a medium round tip; secure the opening with a clip and stand the bag upright.
Use a scale to accurately sift together the confectioner’s sugar and almond flour in a medium bowl and whisk to combine thoroughly.
In a large mixing bowl, combine egg whites, granulated sugar, and cream of tartar, and whisk with a mixer on medium-low speed for 2 minutes.
Increase the mixer speed to medium and continue beating for another 2 minutes.
Add in gel food coloring, if using.
Raise mixer speed to medium-high and beat for 3 minutes more until the meringue forms stiff peaks that hold their shape when the whisk is lifted.
Fold half of the almond flour and confectioner’s sugar mixture into the meringue by scraping the sides of the bowl and folding through the center 15 times.
Add the remaining dry ingredients and gently fold about 30 more times, checking every 5 folds to ensure the batter loosens and flows slightly without being runny.
Transfer the batter to the prepared piping bag, twist the top closed, and remove the clip.
Pipe rounds of batter onto the prepared mats, using circle templates if available, and tap the trays firmly to release air bubbles; pop any remaining large bubbles with a toothpick.
Allow piped shells to rest at room temperature until a dry skin forms, about 20 minutes, using a fan to speed drying if desired.
Preheat the oven to 300°F (150°C) and bake one tray on the middle rack for 18 minutes.
Test shell firmness by gently pressing the sides; if they move, rotate and bake for additional 2-minute increments until firm.
Cool the shells completely before carefully peeling them off the mats.
For the buttercream, beat together salted butter and cookie butter until smooth.
Add powdered sugar and beat until the frosting comes together, mixing for 2 minutes, scrape down the bowl, then mix an additional 2 minutes.
Stir in heavy cream and salt to taste, starting with 1/8 tsp, and beat on low speed to remove air bubbles.
Match shells of similar size in pairs.
Pipe a generous dollop of buttercream onto the center of one shell, leaving space at the edges.
Sprinkle crumbled Biscoff cookies on top of the buttercream.
Add a small dab of buttercream to the matching shell to help it stick to the cookie crumbles.
Press the two shells gently together to form the complete macaron.
Sprinkle any remaining crumbled Biscoff cookies around the sides.
Store macarons in an airtight container in the refrigerator for 24 hours to allow flavors and texture to mature.
