Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan.
In a mixing bowl, combine melted butter, brown sugar, and granulated sugar. Mix well.
Add vanilla extract and eggs to the mixture. Stir until smooth.
Gradually add flour, baking powder, and salt. Mix until just combined.
Pour the batter into the prepared baking pan.
In a small bowl, mix melted butter, brown sugar, and ground cinnamon for the swirl.
Drop spoonfuls of the cinnamon mixture over the batter. Use a knife to swirl it through the batter.
Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Let cool before cutting into squares. Enjoy!