Preheat the oven to 350 degrees F. Grease a total of 20 muffin cups or line with paper cups.
In a medium bowl, stir together the flour(s), baking soda, baking powder, salt, and ginger. In a large bowl, beat the brown sugar, oil, egg, vanilla and buttermilk until smooth.
Add the dry ingredients to the wet ingredients and mix by hand until just combined. Stir in the rhubarb and pecans.
Spoon the batter into the prepared cups. I like to use a large ice cream scoop to evenly and neatly portion.
In a small bowl, stir together the granulated sugar and cinnamon. Mix in the melted coconut oil or butter. Sprinkle about 1/2 teaspoon of this mixture on top of each muffin. I usually start with a spoon and end up using my fingers.
Bake in the preheated oven until the muffins are just cooked through, about 17-20 minutes. All ovens vary a bit so check a minute or two early and cook a few minutes longer if needed. Cool in the pans for 5-10 minutes and then remove to a rack and cook completely.