Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. Crush toffee bars into small bits and set aside.
In a large bowl, combine flour, sugar, brown sugar and butter; mix on low speed with an electric mixer until crumbly. Remove 1/2 cup of crumb mixture and set aside to be used for topping. Add milk, baking powder, vanilla, egg, and 1/2 cup of the crushed toffee bars; beat at low speed until well-mixed. Increase speed to medium, and beat for 1 minute. Spread batter evenly in 9x13 inch pan.
To make the topping: In a small bowl, mix nuts, remaining chopped toffee bars, and reserved 1/2 cup crumb mixture. Sprinkle mixture evenly over batter in pan.
Bake for 30 minutes, or until it tests done. Cool cake completely in pan on rack. Make about 2 1/2 hours before serving, or early the in day.