Step 1

Cut 2 sticks (1 cup) unsalted butter into 1/2-inch cubes and melt in a small saucepan over medium heat. Continue cooking, stirring constantly while scraping the bottom of the pot with a rubber spatula to release any browned bits, until the butter is light brown, smells nutty, and no longer foamy, 3 to 4 minutes.

Step 2

Immediately scrape into a large heatproof bowl. Add 1/4 cup vegetable shortening and stir to combine. Let cool at room temperature for 10 minutes. Meanwhile, drain 1 can sweet potatoes in syrup and place the sweet potatoes in a medium bowl. Smash with the tines of a fork until no large pieces remain and the mixture is mostly smooth.

Step 3

Add 1 cup of the granulated sugar, 3/4 cup packed dark brown sugar, 1 tablespoon vanilla extract, and 1 large egg yolk to the bowl of melted butter. Whisk until completely smooth and glossy. Add the mashed sweet potato (reserve the bowl) and whisk until incorporated.

Step 4

Add 3 1/4 cups all-purpose flour, 1 teaspoon cream of tartar, 1 teaspoon baking soda, 3/4 teaspoon kosher salt, and 1/2 teaspoon baking powder. Stir with a rubber spatula or wooden spoon until the flour is completely incorporated. Cover the bowl with plastic wrap and refrigerate for 15 minutes. Meanwhile, divide the oven into thirds with 2 racks and heat the oven to 350°F. Line 2 baking sheets with parchment paper.

Step 5

Wipe out the sweet potato bowl bowl. Add the remaining 1/2 cup granulated sugar and 1 tablespoon ground cinnamon to the bowl and stir to combine. Scoop out the dough with a 1 1/2-inch wide cookie scoop or into 24 (about 2 rounded tablespoon) portions, and roll into balls. Roll each ball completely in the cinnamon sugar and place at least 2 inches apart on the prepared baking sheets.

Step 6

Sprinkle with more cinnamon sugar. Gently flatten each dough ball with the bottom of a drinking glass until about 1/2-inch thick. Bake until lightly browned and the tops crack slightly, 10 to 12 minutes. Let cool slightly on the baking sheets for 5 minutes, then transfer onto a wire rack to cool completely.