Preheat oven to 325°F. Line a cookie sheet with parchment paper.
Brown the butter: In a medium sized saucepan, add the butter and melt it on medium heat. Once it melts, it will crackle, then become foamy – continue mixing the butter while this happens to prevent burning. The butter will go from melted to golden brown quickly, and you'll know it's almost done when the aroma of browned butter arrives. The whole process should take about 5 minutes! Once golden, remove from heat and transfer to a glass bowl to cool for 20 minutes while you prep the rest of the ingredients.
1/4 cup + 1 tbsp butter
In a large bowl, whisk melted butter and Biscoff spread. Then add in white sugar and brown sugar. Whisk until smooth.
1 tbsp Biscoff cookie spread, 1/3 cup brown sugar, 1 tbsp white sugar
Add in vanilla and egg yolk and continue whisking.
1 egg yolk, 1 tbsp vanilla
Add in flour, baking soda, and salt. Use a rubber spatula and fold the dough until just combined. Do not over mix! Add in crushed Biscoff cookie pieces and semi-sweet chocolate chips and fold into dough.
2/3 cups all-purpose flour, 1/4 tsp baking soda, 1/4 tsp salt, 6 Biscoff cookies, 1/4 cup semi-sweet chocolate chips
Scoop 5 cookie dough balls and place on the prepared cookie sheet. Top off with extra mix-ins on top and a dollop of cookie butter (optional, but makes them look 'prettier' and more bakery-like).
Bake for 13-16 minutes. Mine took 15 minutes. The cookies will be ready when the edges are a golden brown, but the inside will look very soft. Remove from oven and sprinkle a dash of sea salt if desired. Let the cookies cool on the hot pan for a few minutes before serving warm or cool completely and enjoy!