Preheat the oven to 350 degrees F. and grease a 9x13 baking pan.
In a large bowl or mixer fitted with the paddle attachment mix together brown sugar, butter, egg and vanilla extract.
Add in the flour and mix until just combined.
Fold in the walnuts and chocolate chips.
Pour the batter into the greased pan and bake at 350℉ for 20-25 minutes or until the bars have set and are golden brown on the edges.
Cut the cookie bars while they're warm. It’s important to cut the bars while they are warm because they will crack if you cut them later on. They have a delicious crumbly texture!
Notes
Feel free to omit the walnuts, or use any chopped nuts of your choice.
Use the spoon and level method for measuring the flour. If you scoop the flour with your measuring cup, you'll end up with too much, which will cause the bars to be crumbly and dry. Instead, add the flour to the measuring cup by the spoonful, then level it off with a butter knife.
Avoid over mixing the dough. Stir the dough just until you can no longer see large streaks of flour.