Brown the Butter: Melt 1 cup of unsalted butter in a light-colored saucepan over medium heat. Swirl occasionally as it melts, foams, and crackles. After 5 to 7 minutes, the milk solids will turn deeply golden-brown and release a fragrant nutty aroma. Immediately pour into a heatproof mixing bowl, scraping up all brown specks, and let cool for 15 minutes.

Prep the Pan: Preheat oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting. Lightly grease parchment with baking spray.

Mix the Cinnamon Swirl: In a small bowl, stir together melted butter, brown sugar, cinnamon, 1 tablespoon of flour, and salt until a smooth, thick spreadable paste forms. Set aside.

Make the Blondie Batter: Whisk dark brown sugar and granulated sugar into the warm browned butter until glossy. Whisk in eggs, egg yolk, and vanilla vigorously for 1 to 2 minutes until slightly pale and emulsified. Gently fold in flour, baking powder, and salt with a rubber spatula just until no dry flour streaks remain—do not overmix.

Assemble & Marble Swirl: Spread two-thirds of the thick blondie batter evenly into the prepared pan. Drop half of the cinnamon swirl mixture in dollops over the batter. Cover with the remaining blondie batter, followed by dollops of the remaining cinnamon swirl. Use a butter knife or skewer to swirl the batter in a figure-eight pattern.

Bake to Chewy Perfection: Bake at 350°F for 24 to 28 minutes, until the edges are golden and lightly set, the top has a delicate crinkly sheen, and a toothpick inserted into the blondie center comes out with a few moist, sticky crumbs (never wet batter). Do not overbake! Allow to cool completely in the pan on a wire rack.

Glaze & Slice: In a bowl, beat softened cream cheese and butter until creamy. Whisk in powdered sugar, vanilla, and milk until silky and pourable. Drizzle glaze generously over the cooled blondies. Let set for 15 minutes, lift out using parchment handles, and slice into 16 fudgy squares!