Freeze the Biscoff Filling: Line a small plate or baking sheet with parchment paper. Spoon the 1/2 cup Biscoff spread into 10 small portions, about 2 teaspoons each. Freeze for 30–40 minutes, or until firm enough to handle.
Prepare the Cookie Dough: In a large bowl, beat the softened butter, brown sugar, and granulated sugar for about 2 minutes, until creamy. Add the 1/3 cup Biscoff spread and mix until smooth. Add the egg and vanilla extract. Mix just until fully combined.
Add the Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, baking powder, and salt. Gradually add the dry ingredients to the wet mixture. Mix just until a soft cookie dough forms. Fold in the roughly chopped Biscoff cookies. Do not overmix.
Fill the Cookies: Divide the cookie dough into 10 equal portions. Flatten one portion of dough into a thick disc. Place one frozen Biscoff portion in the center. Carefully bring the dough around the filling and pinch the edges closed. Roll gently into a thick ball, making sure the Biscoff filling is completely enclosed. Repeat with the remaining dough and filling.
Add the Topping: Press a few broken Biscoff cookie pieces into the top of each cookie. Arrange the cookies on a parchment-lined baking sheet, leaving plenty of space between them.
Chill: Refrigerate the assembled cookies for 15 minutes. Meanwhile, preheat the oven to 350°F (175°C).
Bake: Bake for 11–13 minutes, or until the edges are set and lightly golden while the centers still look slightly soft. The cookies will continue to set as they cool, so avoid overbaking. Allow them to rest on the baking sheet for 10–15 minutes before transferring them to a cooling rack.
Finish: For an extra Biscoff finish, gently warm 2 tablespoons of cookie butter until pourable and drizzle a small amount over the cooled or slightly warm cookies. For a cleaner stuffed-cookie appearance, leave the tops undrizzled and allow the creamy Biscoff center to be revealed when the cookies are broken open.