Mix flour, cornmeal, sugar, salt, baking powder, and baking soda. Add buttermilk, melted butter, and egg and stir just until incorporated.
Pour 1 1/2 in. oil into a frying pan and heat to about 350° on a deep-fry thermometer. Dust hot dog pieces all over with cornstarch. Using a fork, dip each piece into batter to coat and drop into oil. Fry in batches, turning as needed, until golden brown all over, 3 to 5 minutes per batch. With a slotted spoon, transfer to a paper towel-lined pan and keep warm in a 200° oven until all are done.
Push a cocktail pick into an end of each mini corn dog and serve warm with Cranberry Mustard.
Cranberry Mustard Boil 1 cup unsweetened cranberry juice in a small saucepan until reduced to 1/4 cup. Mix with 3/4 cup Dijon mustard and 6 tbsp. packed light brown sugar. Makes 1 cup.
Note: Nutritional analysis is per piece without mustard.