Once the cupcakes are baked, twist an apple corer down through the top, without hitting the bottom, and burrow out a little tunnel. Then use a piping bag to fill it with extra icing, if you're a sugar fiend, or squirt in some lemon curd, softened peanut butter, jam, or whipped cream.
You can cut off a small bit of the cylinder of cake you take out and use it as a plug, although that's not really necessary. The icing will cover up the hole.
Does anyone else make their cupcakes with a surprise filling? What's your favorite?
Related: Recipe: Classic Buttercream Frosting