Preheat the oven to 400 degrees Fahrenheit. Line a muffin pan with with liners.
In a large bowl, combine the oat flour, cane sugar, baking powder, cinnamon, baking soda, and salt, whisk to combine.
In a medium bowl, whisk together non-dairy buttermilk, coconut oil, eggs, and vanilla extract. Add liquid mixture to the dry ingredients. Stir until combined. Batter will be thick. Fold in rhubarb.
Combine topping ingredients in a small bowl.
Fill muffin cups a little more than ¾ of the way full. Generously sprinkle tops with cinnamon-sugar mixture. Bake for 18-22 minutes or until a toothpick comes out clean. Place muffin pan on a wire rack until muffins cool.
Best when eaten within 2 days. Store in an airtight container.