Place the yuca in a large saucepan with enough cold water to cover by 2 inches. Bring the water to a boil over high heat. Reduce the heat to medium and cook the yuca, uncovered, until it swells and becomes tender, 20 to 30 minutes. Drain the yuca and remove and discard the fibrous core. Mash by passing the yuca through either a food mill or a sieve. Lightly dust a work surface with flour and place 1 cup of the puree on top. Make a well in the center of the puree and put in the flour, butter, egg yolk, and salt. Knead until all the ingredients are well combined. The mixture should not be sticky. If the dough sticks to your fingers, gradually knead in additional flour, 1 tablespoon at a time.

Roll out the dough on a lightly floured surface as thinly as possible. With a sharp cutter, cut out circles 4 inches in diameter; set aside.

Place a rounded teaspoon of picadillo on one side of each of the circles of dough without letting it reach the edges. Moisten the edges of the circles with water. Fold the dough over to form a half circle and press the edges together lightly with the tines of a fork to seal the empanaditas closed.

Heat at least 2 inches of oil in a deep-fryer or a very heavy, deep skillet to 375°F. Preheat the oven to 275°F. Fry the empanaditas, 2 or 3 at a time, until golden brown, 3 to 4 minutes, turning occasionally with a slotted spoon. Drain on paper towels and keep warm on a baking sheet in the oven until ready to serve.