Scoop approximately 1 teaspoon of Biscoff cookie butter per cookie onto parchment paper. Freeze until firm.

Add the cold cubed butter, brown sugar and granulated sugar to a stand mixer. Mix on low for 2–3 minutes, just until the butter breaks down and the mixture begins to come together. Do not cream until fluffy.

Add ¾ cup Biscoff cookie butter and mix briefly until incorporated.

Add the egg, egg yolk and vanilla. Mix just until combined.

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

Add the dry ingredients to the wet ingredients. Mix on low just until a dough forms.

Fold in the broken Biscoff cookie pieces by hand.

Make an indentation in each portion and place one frozen cookie butter center inside. Completely enclose the filling with dough.

Shape each cookie tall and slightly rough rather than perfectly smooth. Press additional Biscoff cookie pieces into the tops.

Refrigerate the formed cookies for at least 2–3 hours or overnight.

Preheat the oven to 375°F.

Arrange the chilled cookies on a parchment-lined baking sheet with plenty of space between them.

Bake for 11–14 minutes, until the edges are set but the centers still look soft.

Let the cookies cool on the baking sheet for at least 10 minutes before moving them.

Sprinkle with flaky sea salt, if desired.