Prep. Preheat the oven to 350°F. Grease a 13 x 9-inch baking dish and set aside. Melt the butter.
Cream the eggs and sugar. In your mixer's bowl, cream the brown sugar and eggs with an electric mixer until smooth and creamy. Add the melted butter to the brown sugar mixture and mix just until combined.
Mix the dry ingredients. In a small mixing bowl, whisk together the flour, baking powder, and salt.
Combine. Using a wooden spoon, gently mix the flour mixture into the brown sugar mixture until just combined. Fold in the chocolate chips and walnuts.
Bake. Pour the mixture into the prepared baking pan and bake for 45 minutes.
Cool and serve. Remove the cookie bars from the oven and let them cool completely before cutting and serving.