01 - Preheat oven to 350°F. Grease a 12-cup muffin tin generously or line with paper liners.
02 - In a medium bowl, toss the diced rhubarb with 1/2 cup sugar and 2 tablespoons melted butter until evenly coated. Spoon about 1 to 2 tablespoons of the mixture into the bottom of each muffin cup, pressing lightly to create an even layer.
03 - In a large mixing bowl, whisk together the flour, 3/4 cup sugar, baking powder, baking soda, and salt until well blended.
04 - In a separate bowl, beat together the cooled melted butter, eggs, milk, and vanilla extract until smooth.
05 - Pour the wet ingredients into the dry ingredients and stir gently with a rubber spatula until just combined. Avoid overmixing to keep the muffins tender.
06 - Divide the batter evenly among the muffin cups, spooning it directly over the rhubarb layer in each cup. Fill each cup about three-quarters full.
07 - Bake for 23 to 25 minutes until the tops are golden brown and a toothpick inserted into the center of a muffin comes out clean.
08 - Let the muffins cool in the tin for 5 minutes. Run a butter knife around the edges of each muffin, then carefully invert the tin onto a wire rack to release the muffins upside down so the rhubarb topping faces upward.
09 - Serve warm or at room temperature. These muffins pair beautifully with vanilla ice cream or fresh whipped cream.