Preheat oven to 375°F. Line an 8 or 9-inch baking pan with parchment paper with overhanging sides.
In the bowl of a stand mixer fitted with a paddle attachment or with a hand-held mixer, beat the butter and sugars on medium high speed until light and creamy, about 3-4 minutes. Scrape down the bowl as needed. Add the egg and vanilla.
In a separate bowl, combine the flour, baking soda, and salt. Mix until incorporated, mixture may be a little crumbly. Mix in the chocolate chips and chopped walnuts.
Press dough into the prepared baking pan and bake until golden brown, 20-30 minutes (depending on pan size). Let cool for about 15 minutes in the pan, then lift bars out of the pan with the overhanging parchment paper and let cool completely on a wire rack. Slice into bars and serve.
Store cookie bars in an airtight container at room temperature for up to one week.
Calories: 307kcal, Carbohydrates: 38g, Protein: 5g, Fat: 16g, Saturated Fat: 8g, Polyunsaturated Fat: 3g, Monounsaturated Fat: 3g, Trans Fat: 1g, Cholesterol: 87mg, Sodium: 174mg, Potassium: 115mg, Fiber: 2g, Sugar: 26g, Vitamin A: 357IU, Vitamin C: 1mg, Calcium: 32mg, Iron: 1mg