Preheat your oven to 350°F (175°C). 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, 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 together the softened butter, granulated sugar, and ground cinnamon for the swirl.

Pour half of the blondie batter into the prepared baking pan and spread it evenly. Drop spoonfuls of the cinnamon mixture over the batter, then pour the remaining blondie batter on top. Use a knife or toothpick to swirl the cinnamon mixture into the batter gently.

Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for at least 15 minutes before cutting into squares.