Preheat the oven to 350 degrees Fahrenheit. Line an 8 x 8-inch pan with parchment paper and set aside.
In a large bowl, add ¾ cup melted unsalted butter, ¾ cup packed light brown sugar, 1 cup old fashioned oats, and 1½ teaspoons vanilla extract. Whisk until well combined.
Add in 1 teaspoon baking soda and 1 cup all-purpose flour. Mix until combined. Take half of this mixture and press it firmly into the bottom of the prepared pan.
Bake for 10-12 minutes or until lightly browned at the edges. Meanwhile, place 1 cup biscoff spread spread in a microwave safe bowl and microwave for 30 seconds.
When the bottom layer is down baking, remove and sprinkle 1 cup milk chocolate chips over it. Pour the biscoff evenly over the chocolate chips.
Sprinkle the remaining oat mixture over top of everything. Bake for another 15-20 minutes or until lightly browned at the edges.
Remove and allow to cool. The cookie butter will be very gooey and soft at first, these bars take a while to be firm -- allow to completely cool before cutting.