Preheat the oven to 350 degrees.

In a large bowl, cream together the butter and sugar until smooth.

Add the egg yolks one at a time, beating after each addition.

Add in the vanilla and mix well.

Stir in the flour until just incorporated.

Place the dough in a large piping bag fitted with a large open star tip.

Pipe the dough onto cookie sheets lined with parchment paper or Silpat. Pipe them in a pinwheels shape until each cookie is round and about 2 inches wide.

Bake for 11-13 minutes or until the cookies just begin to brown.

Allow the cookies to cool for 2 minutes then remove to a wire rack to cool completely.

Place the chocolate in a microwave safe bowl and heat in 30 second intervals, stirring after each heating, until melted.

Dip half of each cookie into the melted chocolate then set on a piece of parchment paper. Top with sprinkles.

Allow cookies to sit for 30 minutes or until chocoalte has hardened.