Position the rack in the lower third of the oven and preheat the oven to 350° F (180 °C). Grease and flour cupcake pan and tap out the excess flour. Or use this pan release spread. Or line cake pans with cupcake liners
Finely crush the Biscoff cookies in a resealable plastic bag or pulse in a food processer. Set aside
mix flour, baking powder, and salt in a medium bowl. Set aside.
In another medium bowl (or a bowl of an electric mixer), beat the butter and sugar on a medium speed until light and fluffy for about 5 minutes, scraping down the sides of the bowl as needed. Add the eggs one at a time and mix well after each addition. Beat in the vanilla until just incorporated.
Reduce speed to low and add flour mixture in three additions, alternating with milk, beginning, and ending with flour. Beat each addition until just incorporated, scraping down the sides of the bowl as needed. Do not overmix. Fold in the Biscoff cookie crumbs
Spoon batter into prepared pan, about 2/3 or ¾ full. Spread the batter, so the centers aren't higher than the sides. Bake until cakes are lightly golden, and a toothpick inserted between the edge and center of the cake comes out clean for about 12 - 15minutes. Transfer pan to wire rack to cool for about 5 minutes.
Transfer the cakes to cool on a cooling rack to cool completely.
In a bowl of a stand mixer or a medium-size mixing bowl, mix the butter and cookie butter until creamy for about 30 seconds.
Add the powdered sugar salt and mix until incorporated. Scrape the bowl with a spatula. Add the vanilla extract and milk, then mix for a minute until fluffy.
Fill a piping bag with a piping tip to decorate the cupcakes with the cookie butter frosting.
Warm a tablespoon of cookie butter and drizzle on the cupcakes. Serve and enjoy.