Preheat the oven to 350˚F.
In a medium bowl, combine the butter, brown sugar, egg and vanilla.
Add the flour, baking soda and salt and stir until almost combined; add the chocolate or nuts (or whatever additions you like) and stir just until blended.
Spread into a parchment-lined 8×8-inch pan and bake for 20-25 minutes, until golden and set.
Serve warm or cool in the pan on a wire rack.