Using a stand mixer with a paddle attachment or hand mixer, cream together 1 cup butter and ⅔ cup granulated sugar for a few minutes. The mixture should be light and airy.
Add 1 egg and 1 tsp vanilla extract. Mix it again until well combined.
Add 2 cups all-purpose flour and ¼ tsp salt and mix on low speed until just combined.
Add 2 teaspoons milk and mix. The dough should be a pipeable consistency but not runny. You can add more milk if needed, but try to use as little milk as possible.
Transfer the dough to a piping bag with a large piping tip.
Pipe 2-inch swirls on a baking sheet (don’t use parchment paper!), 3 inches apart.
Place the baking sheet with piped cookies in the fridge for 15-20 minutes. This extra step will help them keep their shape while baking.
Preheat the oven to 350°F.
Bake the cookies for about 15 minutes or until they are golden brown on the sides.
When done, remove from the oven and let them sit on the baking sheet for about 10 minutes, then transfer to a wire rack to cool off completely.
While they are cooling off, we can prepare the chocolate. You can melt it on a double boiler or in a microwave. Put the chopped chocolate in a medium heat-proof bowl and microwave in 20 seconds increments, stirring after each increment.
Dip ⅔ of the cookies into melted chocolate and place on a parchment paper. Decorate with sprinkles and let them dry off completely.