Heat oven to 375°F.
Place 2 Tbsp butter in 10-inch cast-iron skillet and place in heated oven 5 minutes.
Whisk together milk, melted butter and eggs. Add cornbread mix and stir until combined. Fold in corn and half of cheese.
Spread batter in skillet. Arrange veggies on top of batter and sprinkle with rest of cheese.
Bake 30-36 minutes or until toothpick inserted in center comes out clean, and cornbread is beginning to brown.
Combine avocado, cilantro and sour cream in blender/food processor and puree until smooth. Drizzle over cooked cornbread.