Cream together butter, Biscoff and sugar until light and fluffy.
Add eggs and vanilla.
Add flour, salt and baking powder. Mix until just combined.
Add the milk.
Finish mixing the batter with a spatula until smooth.
Line 12 muffin cups with liners. Divide batter evenly into cups.
Bake for 17 to 20 minutes in a a pre-heated 350°F oven. The top will be firm like a cookie, and the inside will be soft like a cake. Cool completely on a cooling rack.
To make the buttercream, cream together butter and Biscoff.
Slowly add powdered sugar in batches.
Finish adding powdered sugar, along with salt, cream and vanilla. Beat until very light and fluffy.
Pipe frosting onto cupcakes and add decorations if you like.