1) Preheat the oven to 375°F. Lightly grease two baking sheets, or line with parchment.
2) Beat together the butter, sugars, salt, vanilla, baking soda, and baking powder till well combined.
3) Add the egg, beating till smooth. Scrape the sides of the bowl, and beat again till smooth.
4) Add the flour, mixing it in completely.
5) If you're making cookies using chips, nuts, etc, beat in the milk, then the add-ins.
6) If you're making plain cookies, without add-ins, omit the milk.
7) Drop the cookies by the traditional "tablespoonful" (actually about 2 measuring tablespoons) onto the prepared baking sheets, spacing them about 2" apart. A tablespoon cookie scoop works well here.
8) Bake the cookies for 14 to 16 minutes, until they're a light golden brown, and a bit darker around the edges.
9) Remove them from the oven, and cool right on the baking sheets.