In a large bowl, combine melted butter, granulated sugar, and brown sugar until evenly mixed. Add mashed sweet potato, egg yolk, vanilla, and marshmallow fluff and mix until well-combined.
In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Add about half of the dry ingredients to the wet ingredients and mix until just combined. Add the rest of the dry ingredients and mix again until just combined, being careful not to over-mix.
Fold in the white chocolate chips, then refrigerate dough for 30 minutes.
Preheat oven to 350 degrees F and line two large baking sheets with parchment paper. Scoop cookie dough into 1-1/2 tbsp rounds and space at least 2 inches apart on baking sheets.
Bake for 10 to 12 minutes, or until edges start to turn golden-brown.
Let cookies cool on baking sheets for at least 5 minutes before transferring to a cooling rack. Let cool completely before storing in an air-tight container.
Notes
Melted butter: Let the butter cool for at least 10 minutes after melting, so that it's liquid but not too hot.Mashed sweetpotato: We like to mash up leftover baked sweet potatoes for this recipe (we typically peel and cube them, then bake in foil with butter for 45 minutes at 350 - it makes a great side dish!). 1 medium sweet potato will get you about 1/2 cup mashed. But any form of sweet potato puree should work if you want to use your own leftovers or canned ingredients.Mixing: A stand mixer is a game-changer for making this cookie dough! It helps to fully incorporate the sweet potato and marshmallow fluff so the dough has a smooth consistency. If you don't have a stand mixer, you can mix by hand, but note that you might end up with a slightly lumpy texture.Cookiescoop: We use this cookie dough scoop to get consistently-sized cookie dough rounds. Grease it with a bit of cooking spray before using, and the dough should fall right out.Storing: These cookies are a bit cakey, so they will stay soft for a few days in an air-tight container. They're best served within a day or two of baking, so plan accordingly if you're making them for a holiday party or cookie swap! They also tend to stick to each other because of the marshmallow fluff, so we recommend storing them in a single layer, or placing sheets of parchment paper between layers.