Start by melting the butter in a medium saucepan over medium heat. Keep stirring as it foams and then browns, releasing a nutty aroma. Once the butter turns a golden-brown color with small brown flecks, remove it from heat and let it cool slightly.
In a mixing bowl, combine the brown sugar and granulated sugar. Pour in the slightly cooled browned butter and mix well. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon.
Gradually add the dry ingredients to the wet ingredients, folding gently with a rubber spatula until just combined. Avoid overmixing to keep the blondies tender.
Reserve about 2 tablespoons of the browned butter before mixing it with the sugars and eggs. Once your batter is in the pan, drizzle this reserved brown butter on top and use a knife or skewer to swirl it through the batter, creating a marbled effect.
Line your 8x8-inch pan with parchment paper and pour the batter in, spreading it evenly. Bake at 350°F (175°C) for 25–30 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.
Allow the blondies to cool completely in the pan on a cooling rack before slicing into squares. This helps them set and prevents crumbling.