Using paper overhang, remove cookie butter slab onto a plate, peeling the parchment paper off of the slab. Return cookie butter slab to the freezer and place the parchment paper back into the pan. Spray pan with nonstick cooking spray.

Preheat oven to 350°F.

In a medium saucepan, melt the butter over medium-low heat. As the butter melts, it’ll start to foam, then turn a clear golden color. Continue to stir the butter, scraping up any browned specks from the bottom of the pan as it starts to brown. It should have a nutty aroma.

Once browned and nutty, turn off the heat and allow to cool for 5 minutes.

Whisk in the sugar, eggs, and vanilla until smooth. Switch to a wooden spoon, add the flour and baking powder and stir until just combined. Fold in the chopped walnuts.

Pour half of the batter into the prepared pan and spread out into an even layer. Carefully place the frozen cookie butter slab on top of the batter. Pour remaining batter on top, covering cookie butter slab completely.

Bake in the preheated oven for about 30-35 minutes, or until edges are golden brown and there's no jiggle in the center when you gently wiggle the pan. Allow to cool in the pan for 1 hour* before slicing and serving.

Blondies can be stored in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 5 days.