Preheat and Prepare: Preheat oven to 350°F (175°C). Grease a 9×13 inch baking dish or line with parchment paper, leaving overhang on the sides for easy removal.
Make Blondies Base: In a large bowl, cream together melted butter, granulated sugar, and light brown sugar until smooth (about 2 minutes). Beat in eggs one at a time, then stir in vanilla extract until glossy and well-combined.
Add Dry Ingredients: In a separate bowl, whisk together flour, baking powder, and salt. Gradually fold dry ingredients into wet ingredients just until no flour streaks remain. Do not over-mix.
Prepare Cinnamon Swirl: In a small bowl, combine melted butter, ground cinnamon, and brown sugar to form a thick paste (should resemble wet sand).
Layer and Swirl: Spread half the blondie batter into prepared pan. Drop spoonfuls of cinnamon mixture evenly over batter. Use a knife to create swirls in figure-eight motions. Spread remaining batter on top, add remaining cinnamon mixture, and swirl again gently.
Bake: Bake for 25-30 minutes until a toothpick inserted in center comes out with a few moist crumbs (not completely clean). Watch carefully to avoid over-baking.
Cool and Serve: Let cool in pan for at least 10 minutes. Use a sharp knife (wiped between cuts) to slice into 24 squares. Serve warm for gooey texture or completely cooled for firmer blondies.