Sift and mix flour and salt and set aside.
Place softened butter and sugar in a large bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed until pale and fluffy (1-2 minutes). Use a spatula to scrape any ingredients from the side of the bowl.
Add egg yolk and vanilla extract and mix for another 2 minutes.
Slowly add dry ingredients and mix until a semi-soft, sticky dough is formed. If the dough is too thick, mix in 1 tbsp. milk.
Transfer cookie dough to a frosting bag fitted with the open star attachment. Pipe dough either in a circular fashion (for round cookies) or into 1½ to 2" long segments, leaving a 1" space between each one.
Transfer baking sheet to refrigerator and chill for at least 30 mins (up to 1 hr). In the meantime, preheat the oven to 175°C (350F).
Bake for 8-10 minutes or until edges become slightly golden brown.
Allow to cool for 10 minutes in baking sheet (cookies will be very soft when out of the oven), then transfer to a cooling rack to cool completely before coating with chocolate.
Melt chocolate in a microwave safe bowl. (Heat for 30 second increments, stirring with a fork at each pause until chocolate has melted). Stir in coconut oil.
Dip cooled butter cookies into melted chocolate, transfer to a wire rack, and top with sprinkles. Chill for 20 minutes or until chocolate has set.