Preheat oven to 375 degrees F. Line a 13x9 inch pan with foil and spray lightly with cooking spray. If you want to skip lining the pan, feel free.
Mix the flour, brown sugar, softened butter and 1/4 tsp. salt and blend until crumbly. Press into the prepared pan and bake on middle rack for 10 minutes.
Melt the butter in a microwave-safe bowl. Add the remaining salt, butterscotch chips , corn syrup and water to the hot butter and stir well. Microwave at 50% powder for 30 seconds. Stir and repeat if necessary until chips are melted. Note: You may also do that step in a double boiler. The microwave works perfectly, just don't use a high setting. Stir in the nuts (batter will be kind of thick and goopy) and spoon over baked shortbread layer, spreading evenly. Bake for 8 minutes longer. Let sit at room temperature for about 10 minutes, then remove from pan and score while still warm.
Let cool completely before serving.