Preheat the oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

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

In another bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt.

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

Pour the batter into the prepared baking pan and spread evenly.

Bake for 25 minutes or until a toothpick inserted in the center comes out clean.

Allow the bars to cool in the pan on a wire rack.

In a saucepan over medium heat, melt the butter and brown sugar together, stirring until smooth.

Add the milk and bring to a boil. Remove from heat and let cool slightly.

Gradually whisk in the powdered sugar and vanilla extract until smooth and creamy.

Spread the frosting over the cooled pumpkin bars.