Heat oven to 350 Deg F (176 C). Set out baking sheets and line with parchment paper. Fit a large open star piping tip in a piping bag.
In a stand mixer or large bowl with a hand mixer, cream together the butter and confectioners sugar until light and fluffy. About 5-6 minutes. Add in the vanilla and whip for another minute. Whisk together the flour, cocoa powder, cornstarch and salt in a separate bowl then add to the butter mixture and mix until well combined. If using a stand mixer, scrape all the dough off of the beater and remove the bowl form the mixer.
In a separate bowl whip the egg white until it reaches soft peaks. This is when you lift the beater and the peaks fold back over on themselves.
Fold the egg white into the cookie dough with a rubber or silicone spatula until it's completely combined.
Fill the piping bag with a third of the dough and pipe squiggles onto the prepared baking sheets. I made mine with three squiggles you can do how many you like, pipe sticks or circles.
Baked for 10 minutes or until the tops look dry and set. Don’t overtake these.
Let cool for one minute on the baking sheet then carefully transfer to a wire cooling rack to cool completely.
When ready to dip in chocolate, set out baking sheets with wax paper. Make space in your freezer or refrigerator for a tray to set up the chocolate quickly.
Melt the chocolate either in a double boiler ( preferred method) or microwave. Chop the chocolate, place in a bowl and set over a pan of simmering water. Make sure the bottom of the bowl doesn’t touch the water. Stir with a spatula until chocolate is melted and smooth. This can be done while the cookies are cooling.
Dip half a cookie in the chocolate one at a time. Let excess drip off then set the cookie onto the prepared baking sheet. Repeat with a few more cookies before adding sprinkles. Once you have a tray full of dipped and sprinkled cookies, set the tray in the freezer for a few minutes to firm up the chocolate. Repeat with remaining cookies until all are dipped and decorated.
Store cookies in an airtight container either at room temperature or in your refrigerator or freezer.