• Preheat the oven to 375℉/190℃. Line the muffin tin with 10 tulip liners or 12 regular-sized liners and set aside. Wash, dry, and chop the rhubarb into thin slices (about 1/4") and set aside. Melt the butter and allow it to cool as you prep the rest of the ingredients.
  1/3 cup unsalted butter, 1 1/2 cups rhubarb

For the streusel

• In a medium-sized bowl, add in the cold butter, white sugar, and flour. With your hands or a fork, mix together until a crumbly streusel forms. Place in the fridge to chill as you make the muffin batter.
  3/4 cups all-purpose flour, 1/3 cup white sugar, 5 tbsp butter

For the muffins

• In a medium-sized bowl, add in flour, cornstarch, baking powder, and baking soda. Stir to combine and set aside.
  2 cups all-purpose flour, 1 tsp baking soda, 1 1/2 tsps baking powder, 2 tsps cornstarch
• In a large bowl, add in oil, melted and slightly cooled butter, eggs, sugar, sour cream, and vanilla. Whisk for 2-3 minutes until well combined and set aside.
  1/4 cup oil, 1/3 cup unsalted butter, 3/4 cups white sugar, 2 large eggs, 1 tbsp vanilla, 1 cup sour cream
• Add the dry into the wet. With a spatula, stir to form a soft muffin batter. Don't over-mix!
• In the bowl with the sliced rhubarb, add 1 tbsp of flour and evenly mix around to coat the rhubarb pieces. This will prevent them from sinking. Then add into the muffin batter bowl and mix together.
  1 1/2 cups rhubarb
• Scoop the batter evenly amongst the liners. They should be about halfway full in the tulip liners, and almost all the way full if using regular liners.
• Sprinkle a handful of streusel on top of each muffin. Bake for 27-32 minutes if using tulip liners, or 24-27 minutes if using regular liners. A toothpick inserted in the middle will come out clean with no wet batter. Allow to cool and enjoy!