Brown Butter Blondie with Speculoos Buttercream

Ingredients

• 1 cup butter plus 1 tablespoon butter, divided
• 2-1/4 cups packed brown sugar
• 3 large eggs, room temperature, lightly beaten
• 2 teaspoons vanilla extract
• 2-1/4 cups all-purpose flour
• 1-1/2 teaspoons baking powder
• 2-1/4 teaspoons ground cinnamon
• 1/2 teaspoon sea salt
• 1 cup chopped walnuts, toasted
• COOKIE BUTTER FROSTING:
• 2/3 cup butter, softened
• 1/4 cup confectioners' sugar
• 3/4 cup Biscoff creamy cookie spread
• 1 teaspoon vanilla extract
• 1/2 teaspoon ground cinnamon
• 1/4 teaspoon sea salt