In a large bowl, beat together the butter, granulated sugar and brown sugar until combined and creamy - about 1-2 minutes.
Beat in the egg, vanilla extract and almond extract until combined.
Turn off the mixer and scrape down the sides and bottom of the bowl.
Add in the flour, cornstarch and salt. Beat together, starting with the mixer on a low speed and gradually increasing to medium speed until combined.
Carefully fold in the ¼ cup sprinkles using a rubber spatula or wooden spoon. Do not beat in the sprinkles with the electric mixer, as this can cause the sprinkles to bleed color.
Place a large sheet of wax paper on the kitchen counter and spoon/dump the cookie dough onto the wax paper.
Form the dough into a log shape about 1.5 to 2 inches in diameter. Think of packing the dough tightly as you form it into the log. Wrap the log in the wax paper and twist the edges to seal.
Place the log in the fridge for at least 4 hours or up to 5 days. Optionally, place the wrapped log in a freezer bag and freeze for up to 3 months.
When ready to bake, preheat the oven to 350F (180C). If your log is frozen, take it out of the freezer and let it thaw in the fridge overnight the day before you plan to bake. Line cookie sheets with parchment paper or baking mats. Take the log out of the fridge and place on a cutting board. Unwrap the log and slice into cookies about ¼ to ⅓ inch thick with a thin, sharp knife. I get about 30 cookies.
Place the cookies about 1 inch apart on the lined cookie sheets and bake 1 sheet at a time in the preheated oven for about 14-16 minutes or until the tops look set and the edges are starting to turn golden brown. While one cookie tray is baking, return the unbaked cookies to the fridge so that they don't warm up too much.
Let the cookies cool on the cookie sheets.
Finely chop the chocolate. Place the chopped chocolate in a heatproof bowl. Melt in the microwave in 45-second intervals on medium power (not high power, which is often the default) and stir between each interval. Alternatively, melt in a double boiler over gently simmering water.
Dip ½ of each cooking in chocolate. I often use the back of a spoon to simply paint ½ of each cookie. Then place on a wire rack or the cookie tray for the chocolate to harden. Add a few extra sprinkles to the top of each cookie.