Preheat oven to 325°F. Line or grease 6 molds in muffin pan.
In large bowl, mix together almond flour, coconut flour, granular sweetener, baking powder, cinnamon, salt, and xanthan gum (if using).
In medium bowl, whisk together sour cream, butter, eggs, vanilla, and stevia until smooth.
Gently stir the sour cream mixture into the dry mix just until incorporated with rubber spatula to form thick batter.
Stir in rhubarb
Divide batter among muffin cups.
In small bowl, combine the topping ingredients.
Sprinkle sweetened cinnamon mix evenly over each muffin.
Bake muffins until browned and inserted toothpick comes out clean, about 25 to 30 minutes.
Allow muffins to cool on rack in pan for 10-15 minutes before removing.
Serve warm. Leftovers should be refrigerated and heated in toaster oven for 5-10 minutes.