Step 1: Preparation Preheat your oven to 350°F (175°C). Grease an 8×8 inch baking pan with butter or non-stick spray, and line it with parchment paper, leaving an overhang on two sides for easy removal of the bars later.
Step 2: Mix the Cinnamon Swirl In a small bowl, whisk together the melted butter, brown sugar, and cinnamon for the swirl. Set this mixture aside so it can thicken slightly while you prepare the main batter.
Step 3: Create the Blondie Batter In a large mixing bowl, combine the 1 cup of melted butter and 1 ½ cups of brown sugar. Whisk until smooth. Add the egg, egg yolk, and vanilla extract, mixing until fully incorporated and the batter looks shiny.
Step 4: Incorporate Dry Ingredients Gently fold in the flour, baking powder, and salt using a spatula. Mix only until no streaks of flour remain. Be careful not to overmix, as this can make the blondies tough rather than chewy.
Step 5: Assemble and Swirl Spread about two-thirds of the blondie batter into the bottom of the prepared pan. Drop spoonfuls of the cinnamon swirl mixture over the batter. Top with the remaining blondie batter. Use a butter knife or a skewer to gently swirl the layers together to create a marbled effect.
Step 6: Bake Bake for 25 to 30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with just a few moist crumbs. Do not overbake; they will continue to set as they cool.
Step 7: Glaze and Serve Allow the blondies to cool completely in the pan. While cooling, whisk together the powdered sugar, milk, and vanilla for the glaze. Drizzle the glaze generously over the cooled bars. Once the glaze has set, lift the bars out using the parchment paper and cut into squares.