Stir flour, cornstarch, baking powder and salt into a large bowl. Make a well in center; add water, egg yolk and pickle juice, all at once. Stir with a wire whisk to make a smooth batter. Cover bowl and refrigerate for 30 minutes. Heat at least 2-inches of bacon drippings or oil in a deep fryer or large saucepan to 375˚F. In batches, dip pickle slices in the batter to coat evenly and lightly. Fry without crowding in hot oil until golden and crisp, 1-1/2 to 2 minutes. Drain on paper towels and serve at once.