Prepare the cookies: Mix butter and powdered sugar in a mixer for 2-3 minutes into a uniform mixture. Add egg yolks and Biscoff Spread and keep mixing until they are absorbed into the mixture.
In another bowl, mix flour, cornflour and baking powder. Add the flour mixture to the mixer bowl and blend at a low speed just until a uniform dough is formed.
Flatten the dough slightly and wrap in plastic wrap. Refrigerate for at least two hours and up to 24 hours.
Preheat an oven to 340 degrees and line a pan with baking sheet.
Roll out the dough to a thickness of approximately 1/5 inch on a lightly floured work surface (if the dough is slightly stiff or crumbly, you can knead it a little). Cut out 60 cookies with a cutter in 1.5 inch in diameter, lift the dough circles gently with a spatula or a wide knife and move to the pan.
Bake for about 10 minutes until they turn a very light golden hue. Allow to cool completely.
Assemble the Alfajor: Add the Biscoff Spread into a pastry bag. If it is too stiff, heat it for a few seconds beforehand in a microwave to soften it (take care not to completely liquefy it).
Arrange half the cookies so that their flat side faces upward and pipe on their centers ½ a teaspoon of filling. Close with another cookie and press lightly.
You can store in a closed container at room temperature for up to 4 days.