In a large bowl, use an electric mixer to beat the room-temperature butter, ½ cup of Biscoff spread, sugar, brown sugar, vanilla, and salt for 2–3 minutes until smooth and fluffy.
Add the eggs and egg yolk, mixing until the mixture looks creamy.
Add the Biscoff cookie chunks, flour, and baking soda. Mix on low speed just until the flour disappears into the dough. Do not overmix.
Use a spatula to gently fold in the chocolate chips until they are evenly spread out.
Cover the bowl with plastic wrap and chill the dough in the fridge for exactly 1 hour.
Scoop about 1 ½ tablespoons of dough and flatten it in your hand. Place a teaspoon of thick Biscoff spread in the center. Wrap the dough completely around the filling and roll it into a tight, smooth ball so it is fully sealed.
Place the dough balls onto your prepared baking sheet, leaving space between them. Bake for 11–12 minutes until the edges are lightly golden, but the centers are still soft.
Shape and Garnish: As soon as the cookies come out of the oven, press half of a Biscoff cookie into the center of each one.
Immediately place an upside-down cup or large bowl over the hot cookie and gently swirl it in circles to shape it into a perfect, thick round.
Cool: Let the cookies rest on the hot tray for 5 minutes, then move them to a wire rack to cool completely.
Notes
Use a Kitchen Scale: For the absolute best results, weigh your ingredients (especially the flour) using a kitchen scale. Measuring cups can easily pack in too much flour, leading to dry, heavy cookies that won't spread properly.
Watch the Clock, Not Just the Timer: Baking times vary between different ovens because internal temperatures can run slightly hot or cold. Start checking your cookies at around 10 minutes; they are done when the edges are set and lightly golden, even if the centers still look very soft.
Seal the Dough Completely: Make sure there are no cracks or exposed filling when rolling your stuffed dough balls. A tight, smooth seal keeps the cookie butter locked inside so it doesn't leak out onto the pan.
The Cookie Scoot Trick: To get that impressively large, thick, bakery-style shape, swirl an upside-down cup or bowl over the cookies the exact second they leave the oven. This fixes any uneven spreading and pushes the edges into a perfect circle.
Prep the Garnish Early: Have your Biscoff cookie halves ready to go before the timer goes off. The cookies cool and set very quickly, so you need to press the biscuit into the center immediately while the dough is molten.