In a medium light-colored saucepan, melt the butter over low heat. Whisk in the milk powder.
1 1/2 sticks (170 g or 12 Tablespoons) unsalted butter, 2 Tablespoons (18 g) nonfat milk powder
Increase the heat to medium and brown the butter. Whisk the butter continuously and cook it until it bubbles, froths, crackles, and eventually brown bits form in the bottom of the pan. Remove the pan from the stove immediately when the butter turns golden brown. It will continue to cook as it cools in the pan.
While the butter cools, place the pumpkin puree on a paper towel-lined plate. Place another paper towel on top of the pumpkin and press it to absorb excess water. Repeat this 1-2 more times to remove excess water from the pumpkin. Set aside.
1/2 cup (122 g) canned pumpkin puree*
In a small bowl, mix together the sugar and cinnamon for the topping. Set aside. Preheat the oven to 350 degrees.
1 teaspoon cinnamon, 2 Tablespoons (25 g) granulated sugar
Remove 2 Tablespoons of the brown butter and place it in a small mixing bowl. Set aside for the glaze.
Once the butter reaches 75-80 degrees (you can pop it into the fridge or freezer if needed), stir together the remaining butter, pumpkin, brown sugar, egg yolk, and vanilla.
1 egg yolk, 1 teaspoon vanilla, 1 cup (200 g) light brown sugar
In this order, add the flour, cream of tartar, salt, pumpkin pie spice, and cinnamon to the wet ingredients. Using a fork, gently combine the dry ingredients (on top of the wet ingredients), before folding them in. Fold in the dry ingredients.
1 1/3 (173 g) cups flour, 1/2 teaspoon cream of tartar*, 1/2 teaspoon salt, 1 teaspoon EACH pumpkin pie spice & cinnamon
Line an 8 x 8 inch pan with parchment paper. Press the dough into the pan.
Sprinkle the cinnamon sugar on top.
Bake for 18-22 minutes. Cool on a wire rack for about 30 minutes before removing from the pan.
While the bars cool, stir together the butter and cream cheese. Microwave in 5-second intervals, stirring between each until they are melted and combined. The oil might separate, but the powdered sugar will help the glaze come together. Add the powdered sugar and vanilla. Stir to combine. Microwave again in 5-second intervals, stirring between if needed.
2 ounces full fat brick style cream cheese, 1/2 cup (60 g) cup powdered sugar, 1 teaspoon vanilla
Cut the bars into 9 or 12 pieces. Drizzle the glaze onto the cooled bars using a spoon. You can also transfer the glaze to a small baggie and cut the corner to pipe the glaze on. If the glaze is too runny, let it cool, and it will thicken. If it is too thick, microwave it again to thin it out. Store leftover bars in the refrigerator.