Preheat the oven to 425℉/218℃. Line the muffin liners with 12 liners and set aside. Melt the butter and allow it to cool as you prep the rest of your ingredients.
1/4 cup unsalted butter
Wash and dry the strawberries and rhubarb (not completely so it's still a bit wet). Then slice them both thinly, about 1/4" slices and set aside.
1 cup fresh rhubarb, 1 cup fresh strawberries
In a small bowl, whisk together flour, cornstarch, cinnamon, baking powder, and baking soda. Set aside.
2 1/2 cups all-purpose flour, 1 tbsp cornstarch, 1 tsp baking soda, 2 1/2 tsp baking powder, 1/4 tsp cinnamon
In a large bowl, whisk butter, oil, white sugar, eggs, sour cream, and vanilla. Whisk until well combined, for 2-3 minutes.
1/3 cup oil, 1/4 cup unsalted butter, 2 large eggs, 1 tbsp vanilla, 1 cup sour cream, 1 cup white sugar
Add the dry into the wet ingredients and using a spatula, gently fold until almost no dry streaks appear.
In the bowl with the strawberries and rhubarb, add 1 tbsp of flour and evenly distribute it around, coating the fruit evenly.
1 cup fresh rhubarb, 1 cup fresh strawberries
Add the strawberries and rhubarb into the bowl with the batter and mix together until just combined and evenly distributed.
Add the batter into your prepared muffin liners filling them all the way to the top. This is important to get that nice muffin dome we love! Add some extra strawberries and rhubarb and sugar on top if desired.
Bake the muffins at 425℉ for 5 minutes, then lower the temperature to 375℃ and continue baking for 15-20 minutes.
The muffins are ready when a toothpick inserted comes out clean with no wet batter. Once ready, allow to cool and enjoy!
These muffins are best enjoyed day of or within the next 3 days. Store them in an airtight container in your fridge for up to 1 week.