In a large bowl, stir together cornmeal, flour, soda cumin, salt and pepper. Using large holes on a shredder, shred zucchini to make 0.75 cup (packed). Squeeze with your hands to extract excess moisture. In a medium-size bowl, mix egg, buttermilk, oil, zucchini, corn and green onion. Pour liquid into dry ingredients and stir until evenly moistened. Heat a large skillet or griddle over medium-high heat. Grease lightly with butter. Spoon about 2 tablespoons batter onto skillet for each cake, spreading batter to make 8 cm cakes. Cook until tiny bubbles form in the centers and edges of cakes appear dry; turn and cook other sides until browned and cakes puff slightly.
Serve 2 or 3 cakes per person. Top each serving with sour cream as desired.