01 - Preheat oven to 375°F. Grease a standard 12-cup muffin tin generously or line with paper liners.
02 - Combine the diced rhubarb, ½ cup granulated sugar, and 2 tbsp melted butter in a medium bowl. Toss until the rhubarb is evenly coated. Spoon a heaping tablespoon of this mixture into the bottom of each muffin cup, pressing lightly to form an even layer.
03 - In a large bowl, whisk together the all-purpose flour, ½ cup granulated sugar, baking powder, baking soda, and salt until evenly distributed.
04 - In a separate bowl, whisk the melted and cooled butter, egg, buttermilk, and vanilla extract until smooth and well combined.
05 - Pour the wet ingredient mixture into the dry ingredients. Fold gently with a spatula just until combined — stop as soon as no dry streaks remain. Overmixing will produce tough muffins.
06 - Divide the batter evenly among the 12 muffin cups, spooning it directly over the rhubarb layer. Fill each cup about three-quarters full.
07 - Bake for 22–25 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are golden.
08 - Let the muffins cool in the pan for 5 minutes. Run a butter knife around the edge of each muffin to loosen, then carefully invert the pan onto a wire rack so the rhubarb layer ends up on top. Serve slightly warm or at room temperature.