Preheat the oven to 350°F (175°C). Grease an 8×8-inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, whisk together the melted butter and sugar until smooth and well combined.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the baking powder and salt.
Gradually add the flour to the wet ingredients, stirring just until combined. Avoid overmixing.
In a small bowl, combine the sugar and cinnamon for the swirl.
Pour half of the blondie batter into the prepared pan and spread it evenly.
Sprinkle half of the cinnamon-sugar mixture over the batter.
Add the remaining blondie batter and gently spread it over the cinnamon layer.
Sprinkle the rest of the cinnamon-sugar mixture on top, then use a knife to swirl it through the batter to create a marbled effect.
Bake for 25-30 minutes, or until the edges are lightly browned and a toothpick inserted in the center comes out clean.
Allow the blondies to cool completely in the pan on a wire rack before cutting into squares. Enjoy!