Melt butter and chocolate together in a double boiler.
Combine sugar and cornstarch. In a separate bowl, whisk eggs with yolks. Add chocolate mixture to sugar mixture; combine thoroughly. Add eggs and whisk just until smooth. Refrigerate overnight.
Preheat oven to 400 degrees. Line a baking sheet with parchment paper and spray with cooking oil. Spray molds* with oil and line with parchment. Spray parchment with oil. Scoop batter into molds so they are 2/3 full. If using small ramekins, cut a piece of parchment to fit the bottom, then spray the sides generously.
Bake on top rack of oven 20 minutes, or until sides are set but center is still undercooked. Remove from oven. Holding each mold with tongs or potholders, slip a metal spatula underneath and transfer to plate. Slip a butter knife between the parchment and the mold to loosen cake; slip off mold and peel off parchment. Serve immediately. Makes 4 3-inch cakes (if using 8-ounce tomato sauce cans you'll get 6).
*if you don't have bottomless round dessert molds, 8-ounce tomato sauce cans with the tops & bottoms removed work also.