I saw this recipe on a tea company's website. I know people have versions that use graham crackers as a base but this looks even better. I would definitely use dark chocolate for the topping.
Preheat oven to 300°. Grease and line an 8" x 12" baking pan. In a bowl combine 6 oz. flour and 2 oz. sugar. Cut in 4 oz. cubed, cold butter til crumbly and forms a stiff ball. It does take a bit of time. Do not add liquid. When ready, press the dough evenly into the baking tray. Bake for approximately 20 minutes in a cooler than usual oven. When done, set aside to cool.
In saucepan melt 4 oz. butter with 3-1/2 oz. sugar. When the sugar has dissolved, add 2 tblsp corn syrup and 7 oz. condensed milk. Bring the mixture to a boil, lower the heat and simmer for 3 to 4 minutes, stirring constantly. It will thicken and become golden in color. As soon as this happens, spread it over the shortbread and allow it to cool.
In a microwave or over a pan of hot water, melt 7 oz. chocolate (milk or dark, your choice) with 4 tblsp. water. Stir til smooth and pour over the toffee. Let it set for an hour or more before cutting. Lasts about a week in the refrig (if you're lucky).