Open package of buttermilk biscuits, separate them, and place them on a large plate. Let rest 10 minutes. In a 12-inch skillet, heat 1 inch of vegetable oil to 300°F (use a candy thermometer to check the temperature). Carefully place biscuits in skillet. Turn heat off, and cook 1 minute. Turn over using tongs, and cook 1 to 2 minutes more. Transfer doughnuts to a paper-towel-lined plate. Let cool 5 minutes. Cut a small slit in the side of each doughnut, and pipe a dollop of jelly into the center. Sprinkle doughnuts with powdered sugar.