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 granulated sugar and 2 tablespoons melted butter until evenly coated. Spoon approximately 1 to 2 tablespoons of the mixture into the bottom of each muffin cup, pressing lightly.
03 - In a large 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, whole milk, and vanilla extract until smooth.
05 - Pour the wet ingredients into the dry ingredients and fold gently with a spatula until just combined. Do not overmix; a few lumps are fine.
06 - Divide the batter evenly among the muffin cups, spooning it over the rhubarb topping in each well. 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 rest in the tin for 5 minutes, then carefully invert onto a wire rack so the rhubarb topping faces upward. Serve warm or at room temperature.