Preheat the oven to 350 degrees. Grease and flour an 8 x 12 x 2 inch baking pan.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, brown sugar, and granulated sugar on high speed for 3 minutes, until light and fluffy. With the mixer on low, add the vanilla, then the eggs, one at a time, and mix well, scraping down the bowl. In a small bowl, sift together the flour, baking soda and salt and with the mixer still on low, slowly add flour mixture to the butter mixture. Fold the walnuts and chocolate chunks in with a rubber spatula.
Spread the batter into the prepared pan and smooth the top. Bake for 30 minutes exactly. Don't overbake! A toothpick may not come out clean. Cool completely in the pan and cut into bars.