In a large mixing bowl, beat together the softened butter and brown sugar until light and fluffy.
Beat in the egg yolk until fully incorporated, then add the Biscoff spread. Mix until everything is smoothly combined.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing on low speed or folding with a spatula until just combined. Don't overmix - stop as soon as you can't see any dry flour. Gently fold in the crushed Biscoff cookies.
Wrap the dough in cling film and refrigerate for at least 30 minutes.
Preheat your oven to 180°C (160°C fan/350°F). Line two baking trays with parchment paper.
On a lightly floured surface, roll out the chilled dough to about 3-4mm thickness. Use your favorite Christmas cookie cutters to cut out shapes. Place the cut cookies on the prepared trays, leaving about 3cm between each one.Bake for 10-12 minutes, until the edges are lightly golden. Let the cookies cool on the baking tray for 5 minutes, then transfer to a wire rack to cool completely.
Once cooled, gently warm the 150g of Biscoff spread in the microwave for about 10-15 seconds until it's spreadable but not runny. Spread a generous layer on the flat side of one cookie, then sandwich with another cookie. Repeat with the remaining cookies.
