Mix flour, cornstarch and salt with a whisk in a small mixing bowl.

In a medium-sized mixing bowl, beat butter and sugar together (until creamed) using an electric hand mixer, about 1 to 2 minutes. Add vanilla extract and mix thoroughly.

Add dry ingredients in two batches to the butter and sugar mixture, mix until ingredients are combined (with hand mixer) but don't over mix. Add 2 1/2 Tablespoons sprinkles to the cookie dough and mix with rubber spatula.

Split cookie dough in half. Add first half to a long pieces of plastic wrap. Shape into 7-inch logs that are about 1 1/2 inches thick. Once shaped, remove the plastic and grab a new piece of plastic, add 1/2 Tablespoons sprinkles down the length of the plastic wrap, add cookie dough log and add more sprinkles so the outsides of the cookies have enough. Roll up and refrigerate both logs for at least 3 hours or overnight.

Heat oven to 350 degrees F. Cut into 1/2 or 3/4 inch thick slices and place on parchment paper lined baking sheets for 10 to 11 minutes.

While cookies are baking and cooling, melt chocolate and shortening in a microwave safe bowl on 50% power and in increments of 30 seconds at a time, mixing thoroughly between each heating.

Dip cooled cookies into chocolate halfway and allow the excess chocolate to drip back into the bowl. Dip a few cookies and then add sprinkles on top of the chocolate. Chill in the refrigerator to set the chocolate for 15 to 20 minutes.