Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or lightly grease it.
In a large bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
In a separate bowl, mix melted butter, brown sugar, egg, Greek yogurt, and vanilla extract until smooth.
Add the wet ingredients to the dry ingredients and gently mix until just combined—do not overmix.
Fold in the chopped rhubarb evenly.
For the streusel, mix cold butter, brown sugar, flour, and cinnamon with a pastry cutter or fork until crumbly.
Divide the muffin batter evenly between the 12 muffin cups. Sprinkle streusel topping on each muffin.
Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.
Let muffins cool in the tin for 5 minutes before transferring to a wire rack to cool completely.