Preheat your oven to 350°F (175°C) and prepare a 9x9-inch baking pan by greasing it or lining it with parchment paper.
In a large mixing bowl, combine the melted butter with packed brown sugar and granulated sugar. Whisk until smooth and creamy.
Add the eggs and vanilla extract, whisking until well combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Gradually add this to the wet ingredients, stirring gently until just combined.
If using, fold in the white chocolate chips until evenly distributed.
Pour the batter into the prepared baking pan, smoothing the top with a spatula.
In a small bowl, mix together the additional granulated sugar and ground cinnamon, then sprinkle over the top of the batter.
Bake for 25-30 minutes until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Let cool for about 10 minutes in the pan, then transfer to a wire rack to cool completely.
Once cooled, cut into small bites or squares and enjoy!
Notes
For an extra touch, serve with melted chocolate for dipping.