Mix together sourdough starter, flour and water. Mix together well. Let sit on counter, covered, for 3 hours. Then place in the refrigerator for another 3 hours. Set remaining beer aside.
Rinse frog legs and pat dry. Place frog legs in a shallow dish. Add 2 cups buttermilk. Cover. Put in the refrigerator. Let marinade for 3 to 6 hours (or overnight)
Heat 4 inches of vegetable oil in deep fat-fryer or in a deep cast iron dutch oven.
Place flour on a pie plate. Season with seafood seasoning. Whisk together. Take starter out of refrigerator. Whisk in a 1/3 cup beer. Combine well.
Lightly shake frog legs to remove excess buttermilk. Dip frog legs into seasoned flour. Then dip into batter. Drop frog legs into the hot oil. Fry frog legs in oil until golden brown, 5 to 8 minutes. Remove legs from oil with slotted spoon and drain on paper towels.