With an electric mixer, cream the butter and sugar together until they are fluffy. Beat in the egg and the milk/cream.
Add vanilla. Add in the flour and salt at this point. Remove dough from the bowl onto a lightly floured surface.
Knead dough briefly (not too long or it will be a tough cookie) and form into a ball. Wrap with plastic and chill in the refrigerator for about 30 minutes (or you can quick chill in the freezer for about 10).
While dough is chilling, preheat the oven for 350°F Take your mold and lightly brush oil on the baking surface of the cookie molds. Flour the mold(s) and tap out excess flour.
Take a piece of dough and starting at one side, press it firmly into the mold, adding more dough as needed. Be sure to level the back of the cookie.
Now, turn your mold over and strike it a couple of times to release the cookie. The cookie should come right out. Repeat the flour dusting of the mold before molding another cookie. (do not use oil again).
Place the formed cookie on a baking sheet with detail side up and bake in the top 3rd of your oven. This helps highlight the nice detail of the cookie. Bake for about 10 to 12 minutes.
Don't be timid about striking the mold on a cutting board; it will not break if you hold it upright while striking it on its edge.
If the cookie does not release, you have used too much oil. Clean the mold with a stiff dry brush, re-flour, but don't re-oil, and try again.
If your cookies are puffing up and losing detail, use a smaller egg. If they are spreading out and losing detail, use less sugar.
Chill the cookie mold as well to ease in the release of the cookie.
If you are using a mold with multiple sections like the one in the photo, do one cookie section at at time - not the whole mold.