Preheat the oven to 350 F and line a 9-inch square pan with parchment paper.

Combine 1 cup of melted butter and 1 1/2 cups of brown sugar in a large mixing bowl and whisk until smooth. Add the eggs and 1 tablespoon of vanilla and mix until well blended. Stir in the flour, baking powder, and salt just until combined.

Combine 1/4 cup of melted butter, 1/2 cup of brown sugar, the cinnamon, and 1 tablespoon of flour in a small bowl; mix until combined.

Spread half of the blondie batter in the prepared baking pan. Use a spoon to drop half of the cinnamon mixture evenly over the batter layer and gently swirl with a spatula or knife.

Top with the remaining blondie batter and repeat with the remaining cinnamon mixture.

Bake the blondies for 30 to 35 minutes or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs. Cool the blondies on a rack.

Combine the glaze ingredients, adding enough milk to make a drizzling consistency.

Drizzle the glaze over the blondies—slice and enjoy!