In a medium bowl, whisk together the flour, speculaas spice mix, cornstarch, baking soda, and salt and set aside.
In another bowl, cream together 6 oz of the room temperature brown butter and sugars on a low speed until well combined, about 2 minutes. Scrape down the sides of the bowl as needed.
Add the egg and vanilla to the butter/ sugar mixture and beat until combined. Add the dry ingredients to the wet and mix until just barely combined. Gently mix in the butterscotch chips until evenly distributed.
Use a large cookie scoop to portion the dough. Flatten the dough into a disc and place a frozen cookie butter mound in the center. Gently wrap the dough around the filling, making sure the dough ball is well sealed.
Place the cookie balls onto a small plate or board lined with wax paper. Freeze for an hour, then transfer the cookie dough balls to an airtight container or a freezer bag. Then freeze for at least 4 hours, but preferably overnight.
Preheat the oven to 350℉. Line a baking sheet or two with silicone baking mats or parchment paper.
Place frozen cookie dough balls on the prepared baking sheet two inches apart. Keep the remaining dough balls in the freezer.
Bake cookies for 16- 19 minutes. Let them cool on the baking sheet for 5- 10 minutes before transferring them to a cooling rack.