Preheat oven to 375 degrees. Line a 12-cup muffin pan with muffin liners; set aside. In a large bowl, whisk together flour, sugar, baking soda, baking powder and salt.
In a separate bowl, whisk together melted butter, sour cream, milk, vanilla and egg. Add to dry ingredients and gently stir, scraping the bottom, until nearly incorporated with a few floury streaks. Gently fold in the raspberries and rhubarb (reserve a few for topping if you like!).
Spoon batter into prepared muffin tin. Bake for 18-22 minutes, until golden brown and tops spring back when lightly pressed.