Preheat the oven to 375F. Cream the sugar with butter using a small egg whisk, then add the egg and vanilla extract and whisk again. Add the corn starch now, mixing with your hands. Gradually add the flour and baking powder, knead. Let stand for 5 minutes. Make chestnut sized balls with your hands. Put parchment paper on a cookie sheet and arrange the dough balls on it. Place cocoa in a small bowl and pour 1 tablespoon water in another small bowl. Wash and dry a glass soda bottle. Soak the rim of the bottle into the water, then dust it with cocoa. Press the rim of the bottle into each dough ball, making a stem. Wipe the rim of the bottle with tissue paper every 3-4 times. Bake for 25 minutes. Arrange them on a wire rack and allow them to cool.