Heat oven to 400° F. Spray a muffin tin with cooking spray.
Sift flour into a medium-sized mixing bowl. Add baking powder, baking soda, salt, brown sugar, cornmeal, walnuts, oats, and coconut. Stir to combine.
Crack eggs into a large bowl and beat lightly. Whisk in buttermilk and melted butter.
Add dry mixture to the wet mixture, stirring just until blended.
Pour batter into muffin cups, filling each 3/4 full.
Bake approximately 15 minutes until a toothpick inserted into the center of a muffin comes out clean.
Allow muffins to cool in the pan about 10 minutes before removing; serve warm or cool completely before serving.