Preheat your oven to 400°F. Prepare your muffin tins with liners. I love parchment liners.
In a medium bowl, combine your dry ingredients including all purpose flour, granulated sugar, baking powder, baking soda, salt, and cinnamon. Whisk together until well combined.
In a large bowl, combine your wet ingredients including buttermilk, melted butter, eggs, and vanilla extract. Mix until fully combined.
Add the dry ingredients to your wet ingredients a stir until almost combined. Add 1 1/2 cups of finely diced rhubarb and mix until just combined.
You can let your raw muffin batter rest in the fridge for a few hours or even overnight to help get even more rise out of them (optional). As a bonus, your house will smell delicious first thing and you will have an instant breakfast. Add the streusel topping just before baking.
Combine all streusel ingredients in a small bowl . Mix together with a fork or whisk until completely combined and crumbly.
Bake at 400°F for 20-22 minutes or until a toothpick inserted in the middle comes out clean. The tops should be slightly cracked and starting to get golden on the edges.