Preheat oven to 350 F degrees. Line an 8-inch square baking pan with parchment paper, or grease the pan.
In a large mixing bowl combine the butter and brown sugar. Mix with a wooden spoon or stiff spatula until well blended. Add the egg and vanilla. Add the flour and salt and stir well. Set aside 2 tablespoons each of the walnuts and toffee bits. Stir the remaining walnuts and toffee bits.
Spoon the batter into the prepared pan and spread evenly with a spatula. Sprinkle the reserved nuts and toffee bits on top.
Bake for 25 to 30 minutes or until a toothpick inserted comes out mostly clean with moist crumbs. Let the bars cool completely before slicing.
Recipe published in the Weeknight Dessert Cookbook by Mary Younkin