Preheat the oven to 350°F (175°C) and line a 9x9 inch baking pan with parchment paper.

In a mixing bowl, whisk together the melted butter and brown sugar until smooth.

Add the eggs and vanilla extract, mixing until well combined.

In another bowl, combine the flour, baking powder, and salt. Gradually add to the wet mixture, stirring until just combined.

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

Pour half of the blondie batter into the prepared pan and spread evenly.

Sprinkle half of the cinnamon sugar mixture over the batter.

Add the remaining blondie batter on top and swirl with a knife to create a marbled effect. Sprinkle the remaining cinnamon sugar on top.

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

Let the blondies cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract to make the glaze.

Drizzle the glaze over the cooled blondies before cutting into squares.