• Preheat oven to 350 F and line cookie trays with a sheet of parchment paper or silicone baking mat.
• In a large bowl using an electric mixer, or in the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar until light and fluffy.
• Add the egg yolk and vanilla and beat on medium speed until well combined.
• Sift the dry ingredients (flour and salt) into the butter mixture. Beat on low speed until large crumbles form.
• Add the milk and mix well until a dough forms. The dough should not form a ball, but should be very sticky and have the consistency of a thick buttercream. If you stick your spatula in and pull it out, some batter should stick. If the dough is not sticking, add a little more milk.
• Transfer the half of the dough to a large pastry bag fitted with a large star tip.
• For the sandwich cookies, pipe the cookie batter into 2-inch long segments, leaving an inch and a half between each cookie. Use a sharp knife to neatly trim the edges.
• For the star cookies, hold the piping bag vertically and squeeze until your star shape is about 1-inch in diameter. Use a 1/4 teaspoon or your finger to create a divot in the center of each cookie, so that you can pipe some melted chocolate in after baking.
• Repeat with second half of dough.
• Bake cookies in the oven until light golden brown on the edges, about 10-15 minutes.
• Transfer to a wire rack to cool completely.
• While the cookies cool, melt the chocolate in the microwave in a microwave-safe bowl.

To make the sandwich cookies:

• Spread a thin layer of jam onto one side of each cookie. Dip one end of the cookie into melted chocolate and lay it on a piece of parchment paper. Add sprinkles over the top for decoration.

To make the star cookies:

• Spoon the remaining melted chocolate into a piping bag with a small round tip. Pipe dots of chocolate into the little divot in each star cookie. Add some sprinkl