Combine flour, cornmeal, baking powder, 1/2 tsp. salt and 1/4 tsp. pepper in a large bowl.
Combine sour cream, eggs and 1 tbsp. oil in a bowl, stirring until smooth. Add sour cream mixture, corn, and jalapeno to flour mixture; stir just until combined.
Heat a large nonstick griddle over medium-heat heat.
Divide corn mixture into 8 equal portions (about 1/3 c. each); shape into patties. Add patties to griddle; cook 6 minutes on each side.
Combine squash, green onions, basil, vinegar, tomato, remaining 1 tbsp. oil, remaining 1/4 tsp. salt, remaining 1/4 tsp pepper and optional garlic in a bowl; toss. Place 2 corn cakes on each of 4 plates; top evenly with salsa and avocado, if desired.