Before you start : Be sure an adult is nearby to help.
Position the oven racks in the lower and upper thirds of the oven. Preheat the oven to 350°F. Line two 12-cup muffin pans with paper liners.
Make the filling : In a bowl, using an electric mixer, beat together the egg, sugar and cream cheese on medium speed until smooth, about 2 minutes.
Prop up a pastry bag in a sturdy glass or mug with the bag's wide opening at the top. Fold the top of the pastry bag down over the glass. Using a rubber spatula, scrape the filling mixture into the pastry bag. Set the filled pastry bag aside.
Mix the liquids : In a large liquid measuring cup, combine the water, oil, vanilla and balsamic vinegar. (Theres no need to stir these. Just put them together in the cup.) Set aside.
Make the batter : In a large bowl, using a whisk, stir together the flour, cocoa powder, baking soda, sugar and salt until evenly blended. Slowly pour the water mixture into the flour mixture, stirring with a wooden spoon until the batter is smooth.
Pour in the batter: The cupcake batter will be quite runny. If your bowl has a spout, you can pour the batter directly into the muffin cups. Otherwise, it will be easiest to pour the batter into the liquid measuring cup and use it to fill the cups. Fill each lined muffin cup three-fourths full. Do not fill them any fuller than that or there will not be anywhere for the filling to go when the batter rises!
Add the filling : Pick up the pastry bag. Plugging the tip with one hand, twist the top of the bag to close it. Using one hand to guide the tip and the other to squeeze the top of the bag, insert the tip into the center of one of the cupcakes. Gently squeeze a small amount of the cream cheese filling into the batter. You will see the chocolate batter rise as the filling fills the middle.
Keeping in mind that you need to fill 24 cupcakes, squeeze a small amount into each one.
Bake the cupcakes: Sprinkle the top of each cupcake with the chocolate chips.
Put the muffin pans in the oven and bake until a thin skewer inserted into the center of a cupcake comes out clean (ask an adult to help!), 26 to 28 minutes.
Ask an adult to help you remove the pans from the oven and put them on the cooling racks. Let the cupcakes cool in the pans for 20 minutes, then lift out the cupcakes and put them directly on the racks. Let cool completely, about 30 minutes.
