Step 1 Place mushrooms in a small bowl, cover with hot water, and allow to soak until mushrooms have softened, about 30 minutes. Remove mushrooms from the bowl, squeeze to drain, and chop finely.
Step 2 Preheat oven to 350 degrees. Grease a 9×13-inch baking dish with cooking spray.
Step 3 Whisk together flour, cornmeal, sugar, salt, baking soda, black pepper, and cayenne pepper in a large bowl.
Step 4 Melt butter in a large skillet over medium heat. Add chopped mushrooms and stir until lightly cooked and softened, 3 to 4 minutes. Remove from heat and allow to cool for 10 minutes.
Step 5 Stir eggs, buttermilk, and milk into flour mixture. Add mushroom mixture and corn; stir until well mixed. Fold in green onions. Transfer batter to the prepared baking dish and tap on a flat work surface 2 to 3 times to remove any air bubbles.
Step 6 Bake in the preheated oven until golden on top, 30 to 35 minutes. A paring knife inserted into the center should come out clean.
