Preheat oven to 375 degrees. Brush the cups of a standard 12-cup muffin tin with melted butter, and set aside. Whisk together buttermilk, eggs, and sour cream in a medium bowl until combined, and set aside. Whisk together cornmeal, flour, sugar, baking powder, salt, jalapenos, and corn in a large bowl until combined.
With a rubber spatula, fold buttermilk mixture into cornmeal mixture until well combined. Fold in melted butter. Divide batter among muffin cups, filling each three-quarters full. Bake until a cake tester inserted into centers comes out clean, about 25 minutes. Let muffins cool in tin 5 minutes. Turn out into a basket or bowl lined with a clean kitchen towel; cover to keep warm. Serve with butter.