Preheat the oven to 350°F (175°C). Line two cupcake pans or spray with nonstick spray.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, mix Biscoff spread, brown sugar, sour cream, milk, vegetable oil, egg, and vanilla until smooth.

Make a well in the dry ingredients and pour in the wet mixture. Mix gently until just combined.

Fill cupcake liners ¾ full with batter. Bake for 20-23 minutes until a toothpick comes out clean.

Let the cupcakes cool for 5-10 minutes in the pan, then transfer to a wire rack to cool completely.

Beat unsalted butter and Biscoff spread in a bowl until creamy. Gradually add powdered sugar until smooth.

After blending in the sugar, add vanilla extract and a pinch of salt. Adjust consistency with milk if needed.

Frost the completely cooled cupcakes with the Biscoff buttercream using a piping bag or spatula.