Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, whisk together the melted butter and brown sugar until smooth and well combined.
Add the eggs and vanilla extract to the butter-sugar mixture, whisking until fully incorporated.
In a separate bowl, combine the flour, baking powder, baking soda, salt, and cinnamon. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
If using, fold in the chopped walnuts for an added crunch.
Pour the batter into the prepared baking pan, spreading it evenly.
In a small bowl, mix the granulated sugar and remaining cinnamon together. Sprinkle this mixture evenly over the top of the batter.
Bake in the preheated oven for approximately 25-30 minutes, or until the edges are golden and a toothpick inserted into the center comes out clean.
Once baked, let the bars cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.