Line an 8x8 inch baking pan with parchment paper, ensuring a slight overhang for ease of removal. A 9x9 inch pan is also suitable for this recipe.

Crush graham crackers and Biscoff cookies into fine crumbs using a food processor. Transfer the crumbs to a large mixing bowl and incorporate the kosher salt. Set this mixture aside.

Melt unsalted butter and sweetened condensed milk together in a small saucepan over low heat. Once the butter has completely melted, remove the saucepan from the heat.

Pour the melted butter and sweetened condensed milk mixture over the crushed graham crackers and Biscoff cookies. Stir the mixture thoroughly until well combined.

Press this mixture evenly into the prepared baking pan. Place the pan in the refrigerator to chill while preparing the buttercream.

In a stand mixer fitted with a paddle attachment, or using a hand-held mixer, beat the room-temperature butter and cream cheese together until well combined, approximately 3 to 4 minutes. Scrape the sides of the bowl periodically during mixing.

Add the confectioners’ sugar and vanilla extract to the butter and cream cheese mixture. Begin mixing on a low speed to incorporate the sugar, then increase to medium-high and continue beating for an additional 3 minutes until the buttercream is smooth and creamy.

Fold in the crushed Biscoff cookies, mixing for about one minute, or until they are fully integrated into the buttercream.

Spread this buttercream evenly over the chilled Biscoff layer in the baking pan.

Gently warm the cookie butter spread until it reaches a drippable consistency. This can be achieved by heating it in a small saucepan over low heat for less than a minute. If the cookie butter becomes too runny, allow it to cool slightly, which will thicken the consistency.

Transfer the melted cookie butter into a sandwich bag and snip off a tiny corner of the bag. Use this makeshift piping bag to artfully drizzle the cookie butter over the buttercream layer.

Refrigerate the assembled Cookie Butter Bars for a minimum of 2 hours to ensure they are adequately set.

Once chilled, remove the bars from the pan using the overhanging parchment paper and slice them into desired sizes for serving.