Sift together almond flour and powdered sugar twice through a fine-mesh sieve. Discard any large bits of almond. For an extra smooth shell, pulse the mixture in a food processor briefly (about 10 seconds), then sift again. Set aside.

In the clean, dry bowl of a stand mixer fitted with a whisk attachment, beat the aged room temperature egg whites on medium speed until foamy. Add the cream of tartar (if using). Gradually add the granulated sugar, 1 tablespoon at a time, continuing to beat on medium-high speed. Once all sugar is incorporated, increase speed to high and beat until stiff, glossy peaks form. The meringue should be thick and hold its shape when the whisk is lifted. If using, mix in the brown gel food coloring until evenly distributed.

Add half of the sifted almond flour/powdered sugar mixture to the meringue. Fold gently with a rubber spatula until just combined. Add the remaining dry ingredients and continue to fold. This is the 'macaronage' step. You want to deflate some air from the meringue until the batter flows like molten lava or ribbons when lifted, but is not too runny. Test by letting a small amount drop from the spatula; it should settle back into the batter within 15-20 seconds without leaving a peak. Be careful not to overmix.

Transfer the macaron batter to a piping bag fitted with a round piping tip (like Wilton 1A). Pipe 1.25-1.5 inch circles onto silicone baking mats or parchment-lined baking sheets. Leave about 1 inch between each macaron.

Once piped, firmly tap the baking sheets on your counter 2-3 times to release any air bubbles. Use a toothpick to pop any remaining visible bubbles. This helps prevent hollow macarons.

Let the macarons rest at room temperature for 30-60 minutes, or until a skin forms on top. When gently touched, the batter should not stick to your finger. This 'drying' time is crucial for developing the signature 'feet'.

Preheat your oven to 300°F (150°C).

Bake one tray at a time for 12-15 minutes, or until the feet have formed and the shells are stable on their feet (they shouldn't wiggle when gently touched). If the feet appear to be leaning, rotate the tray halfway through baking. Allow the macarons to cool completely on the baking sheet before attempting to remove them. They should peel off easily from the mat once cooled.

In the bowl of a stand mixer with the paddle attachment, beat the softened unsalted butter on medium speed until light and fluffy, about 2-3 minutes.

Add the Biscoff cookie butter and continue beating until well combined and smooth.

Gradually add the sifted powdered sugar, about 1/2 cup at a time, mixing on low speed until incorporated, then increasing to medium. Add the vanilla extract and pinch of salt. If the buttercream is too thick, add heavy cream 1 tablespoon at a time until desired consistency is reached. Beat on medium-high for another 1-2 minutes until light and creamy.

Transfer the filling to a piping bag with a round tip.

Once the macaron shells are completely cool, match them into pairs of similar size. Pipe a generous dollop of Biscoff cookie butter filling onto the flat side of one shell. Gently press the second shell on top to create a sandwich. Repeat with all shells.

If desired, roll the edges of the assembled macarons in crushed Biscoff cookies for extra texture and flavor.

Store the assembled macarons in an airtight container in the refrigerator for at least 24 hours before serving. This allows the flavors to meld and the shells to soften to their ideal chewy texture. Bring to room temperature for about 15-20 minutes before enjoying.