To make the cookies: In the bowl of a stand mixer fitted with the paddle attachment or in a large mixing bowl using a handheld mixer, beat the butter on low speed until smooth. Add the granulated sugar and mix on medium speed for 1 to 2 minutes or until well combined.
Mix in the egg yolks, milk, vanilla extract, and salt until fully combined, stopping to scrape down the sides of the bowl as needed. Then, mix in the flour. The mixture will be a little crumbly, but will come together as you continue mixing it.
Transfer half of the dough to a piping bag fitted with a large piping tip (I used the Ateco 827).
Pipe the dough onto ungreased baking sheets (you may use parchment paper or a silicone baking mat if you prefer) in circles roughly 1.5 to 2-inches in diameter, making sure to leave about 2-inches of room between each one. I typically pipe mine into circles that are about 1.75-inches in diameter. If the dough is too thick to pipe, work the mixture in the piping bag between your hands a little to warm it up.
Refill the piping bag with more dough once you’ve run out and continue piping circles until you’ve used all of the cookie dough.
Meanwhile, preheat the oven to 350°F (180°C).
Place the baking sheets with the piped cookies into the refrigerator and chill for at least 30 minutes while the oven preheats.
Bake for 12 to 16 minutes or until the cookies are set and the bottoms are lightly browned. If you piped the dough directly on the baking sheets, run a thin spatula under the cookies as soon as they come out of the oven so that they don't stick. Allow to cool completely on the baking sheets.
To decorate the cookies: Once the cookies have cooled completely, dip half of each cookie into the chocolate, and place on a parchment lined baking sheet. Add sprinkles on top, if desired. Refrigerate for 30 minutes or until the chocolate is firm. Alternatively, temper the chocolate and let the chocolate firm up at room temperature.
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Notes
Storage Instructions: Cookies may be stored in an airtight container at room temperature for up to one week.Freezing Instructions: Cookie dough will freeze well for up to 3 months. Wrap tightly in plastic wrap and store in a large freezer bag. Thaw the dough to room temperature before piping the cookies.Baked cookies will freeze well for up to 3 months, thaw to room temperature before serving.