In a large bowl, cream the butter & sugar until light and fluffy using a hand mixer. Add the egg yolks and vanilla and mix on medium speed until well combined.
Next, add the flour & salt to the mixture and mix on low speed until large crumbles form. Add the 2 tablespoons of milk and mix until the milk is absorbed.
Pipe the dough into rosettes by using the following motion; holding the bag upright, pipe the dough onto the tray and pull up slightly, then wrap around the center and pull gently to release.