Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a mixing bowl, combine the flour, granulated sugar, brown sugar, baking powder, baking soda, and salt. In a separate bowl, whisk together the melted butter, buttermilk, egg, and vanilla extract. Pour the wet ingredients into the dry ingredients and mix until just combined. Fold in the rhubarb pieces.
In another bowl, mix the streusel ingredients: flour, granulated sugar, brown sugar, and cinnamon. Add the cold, cubed butter and mix until crumbly.
Spoon the muffin batter into the prepared muffin tin and sprinkle the streusel topping over each muffin.
Bake for 20 minutes or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.
Notes
Store any leftover muffins in an airtight container at room temperature for up to 3 days.