Line 2 baking sheets with parchment paper and set aside.

Sift the almond flour and powdered sugar together into a large bowl, discarding any larger bits, and repeat the sifting 1–2 more times for a very fine mixture.

In a clean, grease-free mixing bowl, add the egg whites and beat with an electric mixer on medium speed until foamy.

Gradually add the granulated sugar to the foamy egg whites while beating, then increase the speed to medium-high and whip until stiff, glossy peaks form.

Add the almond flour mixture to the meringue in 2–3 additions, gently folding with a spatula after each addition until the batter flows in thick ribbons and slowly melts back into itself within several seconds.

Transfer the macaron batter to a piping bag fitted with a round tip and pipe small, even circles, about 1 to 1 1/2 inches wide, onto the prepared baking sheets, spacing them slightly apart.

Firmly tap each baking sheet on the counter several times to release air bubbles, then pop any visible bubbles on the surface with a toothpick.

Let the piped shells rest at room temperature until a dry, satiny skin forms on top and they no longer feel sticky when gently touched, about 30–60 minutes.

While the shells rest, preheat the oven to 300°F.

Bake one tray at a time in the preheated oven for 15–20 minutes, rotating the pan halfway through, until the shells rise, develop feet, and lift cleanly from the parchment when gently nudged.

Remove the shells from the oven and let them cool completely on the baking sheets before peeling them off the parchment.

For the filling, place the Biscoff cookie butter and softened unsalted butter in a mixing bowl and beat together until smooth, creamy, and well combined.

In a separate bowl, whip the heavy cream to soft peaks, then gently fold it into the Biscoff mixture until smooth and slightly fluffy.

If the filling is too soft to pipe, chill it in the refrigerator for about 10–15 minutes until it thickens slightly.

Pair the cooled macaron shells by similar size and arrange them in pairs on a work surface.

Transfer the Biscoff filling to a piping bag and pipe a small mound of filling onto the flat underside of one shell from each pair.

Gently press the matching shell on top to create a sandwich, twisting slightly if needed to spread the filling to the edges without cracking the shells.

Place the assembled macarons in a single layer in an airtight container, separating additional layers with parchment if needed.

Refrigerate the macarons for at least 24 hours to allow the shells to mature and the filling to meld with the centers.

Before serving, bring the macarons to room temperature for about 20–30 minutes for the best texture and flavor.