Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a large mixing bowl, combine pumpkin puree, granulated sugar, brown sugar, and eggs. Whisk until smooth.

In another bowl, whisk together all-purpose flour, baking powder, baking soda, cinnamon, and nutmeg.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

Pour the batter into the prepared pan and bake for 25-30 minutes until a toothpick comes out clean.

Allow the bars to cool in the pan for 10-15 minutes, then lift them out and cool completely on a rack.

Prepare the frosting by combining melted butter and brown sugar in a saucepan over medium heat, bringing to a boil for 1 minute.

Remove from heat and stir in powdered sugar until smooth, then frost the cooled bars and let set before slicing.