Preheat the Oven: Start by preheating your oven to 350°F (175°C). Grease and line a 9x13-inch baking pan with parchment paper.
Mix Wet Ingredients: In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Stir until the mixture is smooth and glossy. Add in the eggs one at a time, whisking well after each addition. Finally, mix in the vanilla extract.
Add Biscoff Spread: Gently fold in the Biscoff spread until fully incorporated into the wet mixture.
Combine Dry Ingredients: In another bowl, whisk together the flour, baking powder, and salt. Gradually add this mixture to the wet ingredients, stirring just until combined. Don't overmix!
Pour and Bake: Pour the batter into the prepared baking pan, spreading it evenly. If using, sprinkle chocolate chips or nuts on top. Bake in the preheated oven for about 20-25 minutes or until a toothpick inserted comes out clean.
Cool and Serve: Let the blondies cool in the pan for about 10 minutes before turning them out onto a wire rack. Slice them into squares and serve!
