Line 1 or 2 large baking sheets with parchment paper or a Silpat liner.
In a large bowl, using mixer, cream together the butter and sugar until smooth and fluffy for about 2-3 minutes.
Beat in the egg and vanilla until well combined.
In a medium bowl whisk together the flour, baking soda, baking powder and salt until combined.
Using mixer on low speed, gradually add the dry ingredients to the wet ingredients until the dough comes together.
Add in milk gradually until it's all been combined for a soft dough, by not overmixing.
Preheat the oven to 350 degrees.
Scoop or roll a rounded tablespoon or 1 & 1/2 tablespoons of dough into scoops/balls and place all of them on a prepared baking sheet.
Transfer scoops of dough balls in the fridge for 15 minutes to chill. (Can also cover the dough and chill for 20 minutes, and up to 24 hours.) I prefer to scoop dough balls out before hand, the dough can be hard to scoop when chilled. Plus, you can even freeze the dough balls after they've chilled if you want to bake them later!
When you're ready to bake, place dough balls on prepared baking sheet 2 inches apart. Cookies will spread and puff up while baking.
Bake in preheated oven for 10-12 minutes, if larger cookies, bake longer. Cookies should not be overbaked and look pale-ish colored, but should not be wet on top. (See notes for more info.)
After a few minutes, move the cookies to a wire rack and allow to cool completely before frosting as desired.