Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9×13 inch pan.
In a large bowl, combine brown sugar, flour and butter; mix on low speed with an electric mixer until crumbly. Remove 1 cup of crumb mixture and set aside to be used for topping. To the remainder, add egg, baking soda, milk, vanilla and salt. Batter will be thin; pour into prepared pan. Sprinkle top with chopped walnuts, toffee bits and reserved crumb mixture.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.