Preheat the oven to 190 C (375°F/Gas Mark 5)
Get a baking sheet. Line it with parchment paper or lightly grease it.
Beat the butter, sugars, vanilla, salt, baking powder, and baking soda together. Do this until they're combined.
Add the egg. Beat the mixture until it is smooth.
Add the flour a little at a time. Mix until it is totally combined
[OPTIONAL] If you're mixing stuff in (like chocolate chips,) add them now and mix until they're evenly distributed.
Using a tablespoon, scoop up the batter and drop it onto the baking sheet. Try to put each drop at least 5cm apart. This will prevent the cookies from all joining together.
Put the tray in the oven. Cooking can take anywhere from 10 to 16 minutes. It depends on your specific oven. For best results, cook for 10 minutes and then check every 2 minutes after. The cookies are done when they're golden brown with darker patches at the edge.
Carefully remove the tray from the oven and allow the cookies to cool. Preferably on a wire rack.