In a mixing bowl, combine the dry ingredients from the mason jar.
In a small mixing bowl, combine the cornstarch and water. Stir until mixture turns thin and watery rather than thick and tacky.
In a separate mixing bowl, combine the butter, cornstarch mixture, milk, and vanilla extract.
Slowly stir in the dry ingredients into the wet ingredients. Mix until just combined.
Refrigerate dough for 1 hour.
Preheat the oven to 350 Fahrenheit (176 Celsius).
Lightly grease or line with parchment a cookie sheet.
Use a cookie scoop to make 1 1/2 tablespoon-sized cookie balls. Be sure to leave room between each cookie to account for expansion.
Bake for 13 to 15 minutes until the edges have slightly browned. Remove from the oven.
Reshape the cookies before they have a chance to cool. Then let cool on the pan before serving.