Preheat your oven to 375°F (190°C) and grease or line a muffin tin with paper liners.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
In another bowl, combine the melted butter, egg, vanilla extract, and buttermilk. Mix until smooth.
Pour the wet ingredients into the dry ingredients. Gently fold until just combined, being careful not to overmix.
Add the chopped strawberries and rhubarb to the batter, folding them in gently until evenly distributed.
Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake in the preheated oven for 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.