Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan or line it with parchment paper.

In a large mixing bowl, combine the melted butter and brown sugar. Stir until well combined.

Add the eggs and vanilla extract to the butter mixture, mixing until smooth.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.

In a small bowl, mix the granulated sugar and ground cinnamon together.

Pour half of the blondie batter into the prepared baking pan, spreading it evenly. Sprinkle half of the cinnamon-sugar mixture over the batter.

Add the remaining blondie batter on top, spreading it gently, and then sprinkle the rest of the cinnamon-sugar mixture over the top. Use a knife to swirl the cinnamon-sugar into the batter.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the blondies to cool in the pan for at least 10 minutes before cutting into squares.