In a medium bowl, combine the corn, butter, eggs, sugar, 1 cup of the flour, the salt and the baking soda. Set the batter aside.
Peel the shrimp and place the shells in a shallow saucepan with the water and cayenne. Bring the water to a boil and cook until the liquid is reduced by half, about 5 minutes, then drain the shrimp broth into a medium bowl and set aside. Discard the shells.
Meanwhile, in a medium bowl, toss the shrimp with the Cajun seasoning. In a large nonstick skillet, heat the olive oil over medium heat until it ripples, then sprinkle in the remaining 2 tablespoons of flour and cook, stirring constantly, until the mixture turns rich brown in color, 3 to 5 minutes. Add the garlic and scallions and cook for 2 minutes. Whisk in the reserved shrimp broth and boil the sauce for about 1 minute to thicken. Add the shrimp and cook just until cooked through and pink, about 3 minutes. Remove to a bowl and cover to keep warm.
Wipe the large nonstick skillet clean with a paper towel, coat with cooking spray and place over medium heat. Pour about 1/3 cup of the batter into the hot skillet for each flapjack and cook in batches until the bottoms are golden and bubbles appear on the surface. Flip the flapjacks and finish cooking, about 1 minute more. Repeat with the remaining batter. Top the flapjacks with the shrimp and sauce.