Muffins
Preheat oven to 375°F (190°C). Line or grease a 12-cup muffin tin.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk eggs, melted butter, Greek yogurt, and vanilla.
Fold wet ingredients into the dry ingredients just until combined.
Gently fold in chopped rhubarb. Do not overmix.
Spoon batter evenly into muffin cups, filling about ¾ full.
Streusel Topping
In a small bowl, combine brown sugar, flour, and cinnamon.
Add cold butter and mix with fingers or fork until crumbly.
Sprinkle streusel generously over each muffin.
Bake
Bake for 22–25 minutes, or until golden and a toothpick comes out clean.
Cool in pan 5 minutes, then transfer to wire rack.