Preheat the oven to 150C/130C Fan/Gas Mark 2. Line 2 large baking trays with silicone liners, or greaseproof paper.
In a large bowl, beat the butter for several minutes to soften – best done with a stand/electric mixer. Add the sugar and Biscoff Spread and continue to beat until fully incorporated.
Add the flour and combine again until a crumbly dough forms. Tip onto a lightly floured surface and roll to about 1-2 cm thick. Stamp out 14 or so rounds using a small cookie cutter.
Using small bowls/ramekins, add the milk to one and the Biscoff cookies in the other - crush the cookies to crumbs.
Roll each cookie in the milk first, just getting the edges wet, before rolling in the Biscoff crumbs. Essentially you want a Biscoff crust.
Place on the sheet pans, no need to space out too much as they don't spread. Bake for 15-20 minutes until a light golden colour. Let cool on a wire rack.
Once the cookies are cooled, combine the Biscoff spread and icing sugar in a bowl to form a paste. Add about 1 tsp to half the shortbread cookies, spread around then sandwich together with another cookie.
Repeat with all cookies, until you have about 7 sandwich cookies! Serve immediately or store in an airtight container until ready to eat!
