Mix sugar and cinnamon together in a shallow dish.
Open biscuit tubes, place biscuits on flat surface. Cut holes in center of each biscuit.
Heat oil to 350°. I like to use an electric frying pan with edges. You can also use a deep skillet on the stove, and a thermometer to get the temperature correct.
Place donuts carefully into hot oil, avoiding splashes.
Fry donuts for 1-2 minutes per side, or till golden brown.
Place donuts onto paper towels to drain.
Dip each donut into melted butter, then into sugar/cinnamon mixture.