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

In a bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.

In a large bowl, combine the melted butter, brown sugar, and granulated sugar. Mix until smooth. Add in the eggs and vanilla extract, stirring until fully blended.

Gradually add the dry ingredients into the wet mixture, folding gently until just combined. Be careful not to overmix!

In a small bowl, mix the granulated sugar and ground cinnamon, then drizzle in the melted butter to create a clumpy mixture.

Pour half of the blondie batter into the prepared pan, spreading it evenly. Sprinkle half of the cinnamon swirl mixture on top. Pour the remaining blondie batter and top with the rest of the cinnamon swirl.

Using a knife or a toothpick, gently swirl through the batter to create a marbled effect.

Bake in the oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

While the blondies cool, whisk together the powdered sugar and milk to create a smooth icing.

Drizzle the icing over the cooled blondies, slice, and enjoy!