Add 12 tbsp (1 ½ sticks) of butter to a saucepan over medium heat. Cook, stirring often, until the butter melts, foams, and turns a warm golden brown with a nutty aroma—about 5–7 minutes. Remove from heat and let the browned butter cool 10–15 minutes (you can place in the refrigerator to speed up this process).
Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper. These crisp Biscoff cookie butter cookies bake best on a well-prepared sheet.
In a large bowl, mix the cool browned butter with the melted cookie butter until creamy. Add the white sugar and brown sugar and mix until light and fluffy. Mix in the egg yolks and vanilla bean extract until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, crushed Biscoff cookies, baking soda, cream of tartar, and salt. Slowly add the dry ingredients to the wet ingredients, mixing until a soft dough forms. Avoid over-mixing!
Scoop 16 cookie dough balls onto your prepared baking sheets (I highly recommend putting 8 on each prepared baking sheet to prevent the cookies from overlapping). Bake for 12–14 minutes, until the edges are lightly golden but the centers remain soft and tender. Let cookies cool on the baking sheet for a few minutes before transferring to a wire rack.
For the Cookie Butter Filling, in a medium bowl, beat the unsalted butter and cookie butter together until smooth and creamy. Gradually mix in the powdered sugar, blending until the frosting is thick and fluffy. Add the vanilla extract and milk or cream, 1 tablespoon at a time, until your frosting reaches a spreadable consistency.
Once cookies are completely cool, frost generously. For extra holiday charm, sprinkle with crushed Biscoff crumbs or a white chocolate drizzle.