Banana Walnut Muffins

Ingredients

• 1/2 cup (1 stick) butter, softened
• 1/3 cup granulated sugar
• 1/3 firmly packed light brown sugar
• 2 large ripe bananas, mashed
• 2 large eggs
• 1 teaspoon vanilla extract
• 2-1/4 cups all-purpose flour
• 2 teaspoons baking powder
• 1/2 teaspoon baking soda
• 1/4 teaspoon salt
• 1/2 cup buttermilk
• 3/4 cup chopped walnuts, divided
• 1 cup "M&M's"� Semi-Sweet Chocolate Mini Baking Bits, divided