These dumplings make elegant hors d'oeuvres when served in individual Chinese soup spoons.
Line a strainer with double layer of cheesecloth. Put ricotta in strainer; place strainer over a bowl. Cover; refrigerate overnight. Discard liquid. Transfer ricotta to a medium bowl; add prosciutto, Parmesan, and parsley. Season with salt and pepper; set aside.
Lightly dust a parchment-lined baking pan with cornstarch, and set aside. Place a dumpling skin on a clean surface. Using your fingertip, wet edge of both sides of skin. Place 2 teaspoons of ricotta mixture in center. Draw up edges; make a small pleat. Squeeze pleat firmly together; continue making pleats until dumpling is sealed. Place completed dumpling on prepared pan. Loosely cover with plastic wrap. Repeat process, using remaining skins and filling.
Fill a wok or a high-sided saucepan with water; bring to a boil. Reduce heat to medium, bringing water to a simmer. Using a pastry brush, generously brush top and bottom compartments of large bamboo steamer basket with oil. Brush any excess cornstarch from dumplings; arrange loosely in compartments. Assemble steamer, cover, and place over simmering water. Steam until wrappers are soft and filling is heated through, about 8 minutes, switching top and bottom compartments halfway through steaming.
Pour 1 teaspoon tomato sauce into a Chinese soupspoon, and place a dumpling on top; if using another vessel, serve sauce on side. Garnish with basil leaf, and serve.
Dumpling skins are available in Asian markets and the freezer section of supermarkets.