Assemble Ingredients:
Assemble and measure all ingredients so they are easily accessible.Whisk Dry Ingredients:
In a large bowl, combine the flour, sugar, baking powder, salt, and cinnamon with a whisk until uniform.Mix Wet Ingredients:
In a separate bowl, whisk the eggs, milk, sourdough starter, and melted butter until the mixture is smooth.Combine Wet and Dry:
Carefully incorporate the wet mixture into the dry ingredients, stirring just until combined without overmixing.Heat Frying Oil:
Fill a deep pan with vegetable oil and heat over medium flame until the oil is hot.Fry Doughnuts:
Using a spoon, gently drop portions of batter into the hot oil and fry until each side turns golden brown, approximately 3-4 minutes.Drain on Paper Towels:
Retrieve the doughnuts with a slotted spoon and place them on paper towels to drain excess oil.Coat with Cinnamon Sugar:
While still warm, coat each doughnut in a blend of sugar and cinnamon.