Preheat your oven to 350°F (175°C) and prepare an 8x8-inch baking pan by greasing or lining with parchment paper.

In a large mixing bowl, combine melted unsalted butter with granulated sugar. Mix until creamy and well combined, then add eggs one at a time, mixing thoroughly after each.

Stir in vanilla extract, then mix in baking powder and salt until well integrated.

Gradually fold in the all-purpose flour until just incorporated to avoid overmixing.

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

Spread half the batter into the prepared pan, sprinkle with half of the cinnamon-sugar mixture, then top with remaining batter and finish with the rest of the cinnamon-sugar.

Gently swirl through the batter with a knife or toothpick in a figure-eight motion.

Bake for 25 to 30 minutes, checking for doneness when edges are golden brown and a toothpick comes out with a few moist crumbs.

Allow to cool in the pan for 10-15 minutes, then lift out and cool completely before cutting into squares.