In a large mixing bowl, combine very cold butter, flour, and a pinch of salt until the mixture resembles coarse crumbs. Gradually add cold water until a smooth dough forms.

Wrap the dough tightly in plastic wrap and refrigerate for at least 30 minutes.

Peel and chop sweet potatoes, then boil until fork-tender. Drain and mash; blend in cream cheese, sugar, cinnamon, and nutmeg until creamy.

Dust countertop with flour and roll out dough to about 1/8-inch thickness, cutting out 4-5 inch circles.

Place a generous spoonful of sweet potato filling in the center of each circle. Fold over and seal edges.

Preheat oven to 350°F (175°C). Arrange empanadas on a baking sheet. Brush with egg wash and bake for 25-30 minutes.

Remove from oven and let cool slightly on a wire rack before serving warm.