Preheat the oven to 350°F (175°C) and grease an 8×8-inch baking pan. Line it with parchment paper, leaving some overhang on the sides to make lifting the bars out easier once baked.

In a large mixing bowl, combine the melted butter, cookie butter, and brown sugar using a rubber spatula. Since cookie butter can be thick, you may need to press and fold the mixture until everything is well incorporated.

Stir in the egg and vanilla extract until the mixture is smooth and fully blended.

Add the flour, cinnamon, and salt, and mix until a soft dough forms. The texture should resemble buttery cookie dough.

Transfer the dough into your prepared pan and press it down evenly, making sure it reaches all corners and has a smooth, level surface. Sprinkle the crumbled speculoos cookies over the top and gently press them in.

Bake for 25 to 30 minutes, or until the center looks matte and is no longer sticky to the touch. The bars won’t rise much, and the cookies on top should take on a deep caramel color.

Allow the bars to cool in the pan at room temperature for about 20 minutes, then transfer the pan to the refrigerator for 30 to 45 minutes. This chilling step is important—it helps the bars firm up, as they’ll be too soft to slice cleanly straight out of the oven.

Once fully cooled and set, lift the bars out of the pan using the parchment overhang, slice into squares, and enjoy. The result should be rich, buttery, and wonderfully chewy.