Begin by scooping 13 teaspoon-sized balls of Biscoff cookie butter onto a sheet of parchment paper. Place them in the freezer for about 30 minutes or until solid, which will help maintain their shape when placed inside the cookies.

While the cookie butter balls freeze, preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a medium mixing bowl, combine the room-temperature butter and ½ cup of Biscoff cookie butter. Use a mixer to cream these together on medium speed until smooth, about 2-3 minutes.

Next, add both brown and granulated sugar to the creamed mixture. Beat until light and fluffy, about another 2-3 minutes. Mix in the room-temperature egg and vanilla extract until fully incorporated.

Carefully sift in the all-purpose flour, baking soda, and salt. Stir gently with a spatula until just combined.

Using a cookie scoop, form large dough balls from the mixture, then take one of the frozen cookie butter balls and press it into the center of each dough ball.

Space out 6-7 cookie dough balls on each prepared baking sheet, leaving room for spreading. Bake for about 11-13 minutes, until edges are golden brown.

Drizzle melted Biscoff cookie butter over the warm cookies and sprinkle with flaked salt. Allow cookies to cool on the baking sheets for 5-10 minutes before transferring to a wire rack.