Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon (if using).
In a large bowl, cream together the softened butter and sugar until light and fluffy.
Add the Greek yogurt, eggs, and vanilla extract, mixing until well combined.
Gradually add the dry ingredients to the wet mixture, stirring just until combined.
Gently fold in the chopped rhubarb.
Divide the batter evenly among the muffin cups, filling each about two-thirds full.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.