Line an 8X8 square pan with parchment paper, leaving an overhang so you can lift the bars up when they are finished.
In a medium bowl, combine all dry ingredients (flour, spices, baking powder, and salt). Mix lightly with a fork or whisk until combined and set aside.
Preheat the oven to 350F. In a large bowl, combine melted butter, brown sugar, and white sugar. Mix with a nonstick spatula until combined. Add in pumpkin and mix until no lumps of pumpkin remain and mixture is smooth.
Add in the egg, egg yolk and vanilla and mix thoroughly making sure that all ingredients and homogenous.
Fold in all dry ingredients and the chocolate chunks. Mix just until combined and thick batter forms.
Spread batter into prepared pan and bake for 30-35 minutes. The bars are done when the edges are set and browned and the center looks slightly underdone. If they are overcooked they will become cakey, so for gooey bars, err on the side of less cooking time!