Preheat your oven to 350 degrees Fahrenheit and prepare a 12-cup muffin tin with paper liners.

Add the flour, baking powder, salt, sugar and cinnamon to a large bowl and whisk together to combine. Add the rhubarb and toss well to coat.

Measure the milk and coconut oil into a large measuring cup, adding the eggs and whisking until all the wet ingredients are combined.

Add the wet ingredients to the dry ingredients and mix just until no streaks of flour remain.

Combine all the ingredients of the streusel topping and mix with a fork until crumbly. Start with 4 tablespoons of butter and then add one more if necessary to create a crumbly texture.

Distribute the muffin batter evenly among the 12 prepared muffin cups and top each with the streusel topping.

Bake at 350 degrees Fahrenheit for about 20 minutes or until the streusel topping is a light golden brown and a toothpick inserted into one of the muffins comes out clean.