01 - Preheat the oven to 375°F (190°C). Grease a standard 12-cup muffin tin generously or line with paper liners.
02 - In a medium bowl, combine the diced rhubarb, ½ cup granulated sugar, and 2 tbsp melted butter. Toss until the rhubarb is evenly coated. Spoon a heaping tablespoon of the mixture into the bottom of each muffin cup, pressing lightly to form an even layer.
03 - In a large bowl, whisk together the flour, remaining ½ cup sugar, baking powder, baking soda, and salt until well blended.
04 - In a separate bowl, whisk together the melted and cooled butter, egg, buttermilk, and vanilla extract until smooth and homogeneous.
05 - Pour the wet ingredients into the dry ingredients. Using a spatula, fold gently just until combined. A few lumps are fine — do not overmix, as this will yield tough muffins.
06 - Divide the batter evenly among the prepared muffin cups, spooning it directly over the rhubarb layer in each cup. 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 knife around the edges of each muffin to loosen, then carefully invert the pan onto a wire rack. The rhubarb layer should now be on top. Serve slightly warm or at room temperature.