01 - Preheat the oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease the wells.
02 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, beat the eggs, then whisk in melted butter, buttermilk, and vanilla extract until smooth.
04 - Pour the wet ingredients into the dry ingredients and gently fold until almost combined. Do not overmix.
05 - Gently fold in the diced strawberries and rhubarb just until evenly distributed throughout the batter.
06 - Divide the batter evenly among the muffin cups, filling each about 3/4 full.
07 - Sprinkle the tops with coarse sugar if desired for a crunchy finish.
08 - Bake for 22 to 25 minutes, or until a toothpick inserted in the center comes out clean.
09 - Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.