01 - Preheat the oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease the wells with butter or cooking spray.
02 - In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until thoroughly blended.
03 - In a separate medium bowl, beat the eggs, then whisk in the melted and cooled butter, buttermilk, and vanilla extract until smooth and well combined.
04 - Pour the wet ingredients into the dry ingredients. Gently fold with a spatula until just barely combined—some dry streaks should remain. Do not overmix as this will create tough muffins.
05 - Gently fold in the diced strawberries and rhubarb until evenly distributed throughout the batter.
06 - Divide the batter evenly among the 12 muffin cups, filling each approximately 3/4 full.
07 - Sprinkle coarse sugar over the tops of each muffin for a crunchy sweet crust, if desired.
08 - Bake for 22 to 25 minutes, or until a wooden toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs.
09 - Let the muffins cool in the pan for 5 minutes, then carefully transfer them to a wire rack to cool completely before serving.