Preheat your oven to 350°F (175°C). Prepare an 8x8 inch baking pan by greasing it lightly and lining the bottom and sides with parchment paper for easy removal.
In a large mixing bowl, combine the melted butter, packed brown sugar, and granulated sugar. Using a whisk, blend them together until the mixture is smooth and well-combined, with no lumps.
Crack in the eggs, one at a time, and mix thoroughly after each addition to ensure they are fully incorporated. Stir in the vanilla extract for a delightful aroma.
In a separate bowl, sift together the all-purpose flour, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, stirring gently until just combined. Be careful not to overmix; a few flour streaks are perfectly fine.
With a spatula, gently fold in the Biscoff spread, ensuring it is evenly distributed throughout the batter. If you're using chocolate chips, add them in now, folding gently to incorporate without overworking the batter.
Pour the luscious blondie batter into the prepared baking pan. Use a spatula to smooth the top, creating an even layer.
Drizzle additional Biscoff spread across the batter's surface. You can use the back of a spoon to create beautiful, artistic swirls for visual appeal.
Bake in the preheated oven for 25-30 minutes, or until the edges are a golden brown, and the center remains delectably gooey. A toothpick inserted into the center should come out with a few moist crumbs clinging to it.
Once baked, allow the blondies to cool in the pan for at least 15 minutes. Carefully lift them out using the parchment paper and transfer to a wire rack to cool completely.
Notes
For an indulgent treat, serve warm with extra Biscoff spread and vanilla ice cream.