Preheat the oven to 400 degrees. Line a muffin tin with 12 muffin liners.
Mill enough flour for 1 and 3/4 cups of flour.
Combine the flour with the 1/2 cup coconut sugar, baking powder, and salt.
In another bowl, mix the egg, greek yogurt, milk, butter, and vanilla.
Mix the dry ingredients into the wet ingredients.
Add in the rhubarb but be careful not to overmix.
Divide the muffin batter into the 12 muffin tins.
To make the crumble topping, mix the oats, 1/4 cup coconut sugar, and butter in a mixer.
Top the muffins with the crumble.
Bake for 20-25 minutes.
Let rest in pan for 5 minutes, then transfer to a cooling rack to let cool.