1cupsemi-sweet chocolate chips, butterscotch chips or chopped nuts, such as walnuts or cashews (or use a combo of these add-ins)
Preheat the oven to 350°F. Grease a 13x9-inch baking pan, or spray with nonstick spray.
In a medium-sized saucepan, melt the butter over low heat. Remove from the heat, add the brown sugar, and stir until blended. Let cool for at least 5 minutes. Add the vanilla, then add the eggs, one at a time, and stir to mix quickly until well combined, so the eggs don’t cook in the still slightly warm brown sugar mixture.
In a small bowl combine the flour, baking soda, and salt. Add to the saucepan and stir until combined. Stir in the nuts or chips, if using, and spread into the prepared pan. Smooth the top with a spatula. Bake until a wooden skewer or a toothpick inserted in the middle comes out clean, 24 to 28 minutes.
Cool them in the pan on a wire rack. When cool, cut into 24 squares.