1. In a mixing bowl, blend together butter until smooth and creamy. Add in eggs and mix well. Slowly pour in the cake mix and additional flour until well-incorporated. The batter should be thick.
2. Heat vegetable oil in a heavy-bottom frying pan to 375°F. Using a small cookie scoop, form the batter into 1" balls and carefully place in oil. (Add up to 6 in each batch.) Fry for about 4-5 minutes on each side, until golden brown. Use a heat-proof slotted spoon to remove from oil, and place on a parchment lined cookie sheet to cool.
3. Sprinkle powdered sugar over the doughnuts. Pipe 1-2 teaspoons jelly into the center of each doughnut. Serve immediately, and enjoy!