Special equipment: a pastry cutter and a 3- or 4-inch round or fluted cookie cutter
Preheat the oven to 325 degrees F. Line baking sheets with parchment paper or baking mats.
Cream the butter and sugar together. Sift in the flour and cornstarch and blend using a pastry cutter until the ingredients all come together. (If the mixture seems excessively dry, cut in 1 more tablespoon of butter--but only if you really need it.) Form the dough into a big ball, wrap in plastic wrap and refrigerate 20 minutes.
Roll onto a lightly floured surface and cut into rounds with a 3- or 4-inch round or fluted cookie cutter. Place on the prepared baking sheets. Bake for 20 minutes. Do not allow the shortbread to brown; check your oven sooner if it runs hot. Let cool.
Melt the chocolate in a bowl over a pan of boiling water, or in a microwave. Once the shortbread cookies have cooled, spoon a little chocolate onto each cookie, spread it out and sprinkle over some sprinkles! Allow the chocolate to harden for an hour before eating, storing or packaging.