Preheat the oven to 350F. In a small saucepan, melt the butter over medium-high heat. Keep it on the heat, swirling the pan occasionally, until the foam starts turning golden and the mixture smells nutty. Remove from heat and pour into a mixing bowl.
Stir in the brown sugar, then the 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 an 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.