Preheat oven at 350-degrees. Line a 13x9 pan with foil and apply nonstick spray or butter.
To make the cookie layer, cream together the butter and brown sugar. Once they are smooth, add the eggs and vanilla. Next, mix in the dry ingredients: flour, baking powder, salt, and cream of tartar. Spread the mixture evenly in the pan.
Next up is the pumpkin pie layer. No need to wash the bowl. Cream together the butter and sugar followed by the eggs and vanilla. Add the baking powder, salt, pumpkin pie spice, and flour. Once they are just mixed, fold in the pumpkin puree. Spread the everything over the cookie layer.
Spoon together the topping and add it evenly over the top of the dough.
Bake for 38 to 41 minutes, until toothpick inserted in the center comes out clean (though it may still be softer when cut). Cool at room temperature at least an hour, then chill in fridge for several hours. Use the foil to lift the contents onto a cutting board to slice up.
Keep stored chilled in a sealed container between waxed paper layers. Best consumed within 2 days; bars get mushier after that, but are still tasty.
OM NOM NOM!