This comes courtesy of Cooks Illustrated.  This British cookie is a shortbread crust with a layer of caramel and a veneer of chocolate to top it off.  This has become another Christmas tradition that is worth breaking out the digital thermometer for.
Crust:
2 1/2c flour
1/2c sugar
3/4t salt
1c  butter, melted

Caramel:
14 oz can sweetened condensed milk
1c brown sugar
1/2c cream
1/2c corn syrup
1/2c butter
1/2t salt

Chocolate Top:
8 oz bittersweet chocolate

Preheat oven to 350 F, and adjust oven rack to lower middle position.  Line 9 by 13 baking pan with foil.  Combine flour, sugar and salt in a medium bowl and add melted butter.  Stir to combine so that no dry flour remains.  Pour mixture into the baking pan and evenly press to create a uniform layer.  Using a fork, poke holes in the crust every inch or so.  Bake until golden brown, about 25 minutes.  Once the crust is baked, press with a flat object to compress it.  Let cool for 20 minutes.

Combine all ingredients for the caramel in a large saucepan.  Cook over medium heat with constant stirring until the mixture reaches 236 F.  Immediately remove mixture from heat and pour over crust and allow to cool completely, 1~2 hours.

Microwave 6 oz of the chocolate until just melted, stirring every 15 seconds.  Add remaining 2 oz of chocolate and stir until melted.  Spread chocolate over caramel layer evenly and refrigerate for 10 minutes.

Cut cookies with a serrated knife.  Recipe will generate about 40 cookies.