Preheat oven to 350°F (175°C).
Line a cupcake pan with liners and set aside.
In a food processor, grind Biscoff cookies into fine crumbs.
Add melted butter, brown sugar, and cinnamon.
Pulse until combined. Set aside.
In a large bowl, beat butter, granulated sugar, and brown sugar for 2–3 minutes until light and fluffy.
Add eggs one at a time, mixing well after each addition.
Mix in vanilla extract.
Add ⅓ of the flour, baking powder, baking soda, and salt. Mix.
Add ½ of the milk and mix.
Add another ⅓ of the flour, mix.
Add remaining milk, mix.
Finish with remaining flour and mix just until combined.
Add 1 teaspoon of cookie crumbs to the bottom of each liner.
Add 1 scoop of cupcake batter.
Add another 1 teaspoon of cookie crumbs.
Top with a final scoop of batter.
(Each cupcake uses 2 teaspoons crumbs + 2 scoops batter.)
Use a toothpick to gently swirl the layers—do not overmix.
Bake for 15–17 minutes, or until a toothpick comes out clean.
Remove and cool completely on a wire rack.
Beat butter for 3–4 minutes until pale and fluffy.
Mix in vanilla and cinnamon.
Gradually add powdered sugar until smooth and creamy.
Transfer frosting to a piping bag and frost cooled cupcakes.