Preheat oven to 350°F. Spray a 13x9 inch pan with cooking spray.
Cut the butter into ½ inch pieces.
Place the flour, brown sugar, and butter in a large bowl. Mix with a fork until a shaggy dough forms. Reserve 1 cup for the topping.
In another bowl, whisk together the egg, vanilla, and milk.
Add the salt and baking soda to the remaining brown sugar mixture. Stir to combine. Stir in the milk mixture. (There will be lumps of butter—that is ok!)
Pour the batter into the prepared pan.
Mix the reserved crumbs with the walnuts and Heath toffee bits. Sprinkle over the cake batter.
Bake for 35–40 minutes, or until a toothpick comes out clean. Let cool before serving with whipped cream.
Notes
I used Heath Bits O' Brickle. If you'd like the cake to have chocolate, you can substitute 6 finely chopped Heath Bars in place of the toffee bits OR Heath now makes English toffee bits covered in chocolate (in the baking aisle, they're just Heath bars chopped up for you). Store the cake for 4–5 days tightly covered at room temperature. The crumble on top will get slightly softer as it sits.You can freeze any leftover cake, tightly wrapped, for up to two months.