Preheat oven to 350°F (175°C). Line a muffin pan with paper liners.
In a medium bowl, whisk together the spelt flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
Cream the butter and brown sugar in a large mixing bowl until light and fluffy. Add the eggs, one at a time, beating well in between each addition.
With a wooden spoon, stir in the mashed bananas, buttermilk, and vanilla. Slowly stir in the dry ingredients just until incorporated. Do not over stir. Gently stir in the chopped rhubarb.
Spoon the batter into the liners filling each cup 3/4 of the way full. Dot top with more chopped rhubarb.
Bake the muffins for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and let cool 5 minutes in the pan. Then transfer the muffins to a wire rack to cool completely.