Line a small baking sheet or plate with parchment paper. Divide the Biscoff spread into 9 portions, approximately 1 heaping tablespoon or 20 grams each. Freeze for at least 30 minutes, or until completely firm.

Add the butter to a light-colored saucepan over medium heat. Cook, stirring frequently, until the butter foams and the milk solids at the bottom become golden brown. Once it smells nutty and toasted and is browned, immediately pour it into a large heatproof mixing bowl. Let it cool for 10–15 minutes. It should remain liquid but should not feel hot.

Add the 2 tbsp of Biscoff spread, brown sugar, and granulated sugar to the browned butter. Whisk until thoroughly combined.

Add in the egg, egg yolk, whole milk, and vanilla. Whisk until the mixture looks smooth, thickened, and glossy.

In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt.

Add the dry ingredients to the wet ingredients. Fold everything together just until a soft dough forms. Fold in the chopped Biscoff cookies.

Divide the dough into about 9 equal portions, roughly ⅓ cup each.

Flatten one portion of dough into a disc. Place one frozen portion of Biscoff filling in the center. Bring the dough around the filling and pinch it closed, making sure the filling is completely sealed.

Shape each cookie into a thick disc approximately 1 inch tall. Do not shape the dough into tall mounds, as this can prevent the cookies from spreading. Press additional Biscoff pieces onto the tops.

Place the assembled cookies in the refrigerator for 30 minutes. Meanwhile, preheat the oven to 350°F (177°C). Line a light-colored baking sheet with parchment paper.

Arrange 4 to 6 cookies on the prepared pan, leaving several inches between each one. Bake for 13–15 minutes. The edges should be golden and set, while the centers should look soft and slightly underbaked.

If needed, use a large round cookie cutter to gently shape the cookies immediately after baking. Allow them to cool on the baking sheet for at least 15 minutes before moving them.

Finish with warmed Biscoff spread, additional cookie pieces, and a small pinch of flaky salt.