Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper.

In a large bowl, whisk together the flour, old-fashioned oats, brown sugar, 1 teaspoon cinnamon, baking powder, and 1/2 teaspoon salt. Cut in the cold butter until the mixture resembles coarse crumbs. Press two-thirds of this mixture firmly into the bottom of the prepared baking pan.

Bake the crust for 15 minutes until set. Let cool slightly.

While the crust cools, make the filling. In a medium bowl, whisk together the eggs and granulated sugar until well combined. Add the pumpkin puree, whole milk, pumpkin pie spice, 1 teaspoon vanilla extract, and 1/4 teaspoon salt. Whisk until smooth.

Pour the pumpkin filling over the warm crust. Sprinkle the remaining third of the oat mixture evenly over the top.

Bake for an additional 25-30 minutes, or until the filling is set and the top is golden. Let the bars cool completely in the pan, then refrigerate for at least 2 hours until chilled.

Prepare the frosting. Beat the softened cream cheese and softened butter together until smooth. Add 2 1/2 cups powdered sugar, 1 teaspoon vanilla extract, and a pinch of salt. Beat on low until combined, then increase to high and beat until fluffy.

Lift the chilled bars out of the pan. Spread or pipe the cream cheese frosting generously over the bars. Dust with cinnamon and cut into squares.