You can twist off mushroom stems with your hands and then scrape them out, but this can be messy, and the inside of your mushroom will have a craggy, uneven surface.
The key to using a melon baller to prepare mushrooms is to remove their stems by hand before using the edge of the baller to scoop out the inner gills of the cap.
Use the baller to scoop in the filling. It works perfectly with all sizes of mushrooms, even small ones like buttons or chestnuts, which make fantastic bite-sized hors d’oeuvres.