1Mix sugar and cinnamon together and set aside.
Mix all remaining dry ingredients in a medium bowl.
2Whisk sugar and butter in a large bowl. Add egg and buttermilk. Whisk until smooth. Gently stir in dry ingredients to form a soft dough. Cover and refrigerate for at least 30 min. Can also refrigerate overnight.
3When ready to fry, heat an inch of oil in frying pan on med-high. Around 325 degrees. Using a small scoop or 1 teaspoon measuring spoon and scoop the dough into balls. drop dough balls into hot oil. About 1-2 min should be golden brown on one side then flip and continue frying 1-2 min on the other side.
4Drain on paper bags and start next batch to fry. Quickly dredge hot doughnuts in the cinnamon sugar mixture. Serve warm or room temp.