Candied Peanuts: Line a large baking sheet with parchment paper; set aside. Put sugar and water in a 2-quart heavy-bottomed sauce. Cook, over medium-high heat, stirring until the sugar dissolves. Keeping the heat fairly high, continue to cook the sugar, without stirring, until it just starts to color. (If sugar splatters onto the sides of the saucepan, wash down the splatters with a pastry brush dipped in cold water.) Toss in the peanuts and immediately start stirring. Keep stirring, to coat the peanuts with the sugar. Within a few minutes, they will be covered with sugar and turn white – keep stirring until the sugar turns back into caramel. When the peanuts are coated with a nice deep amber caramel, remove the pan from the heat and turn the nuts out onto the prepared baking sheet, using the wooden spoon to spread them out as best you can. Cool the nuts to room temperature. When they are cool enough to handle, separate the nuts or break them into small pieces. Divide the nuts in half. Keep half of the nuts whole or in biggish pieces for the filling, and finely chop the other half for the topping.
Crust: Center a rack in the oven and preheat the oven to 350 degrees F. Butter an 9-inch square pan; line with parchment paper. Set aside.
Toss the flour, sugar, confectioners' sugar and salt into a food processor and pulse a few times to combine. Toss in the pieces of cold butter and pulse about 12 times, until the mixture looks like coarse meal. Pour the yolk over the ingredients and pulse until the dough forms clumps and curds – stop before the dough comes together in a ball.
Turn the dough into the buttered and parchment-lined pan and gently press it evenly across the bottom of the pan. Prick the dough all over with a fork and slide the sheet into the oven.
Bake the crust for 15 to 20 minutes, or until it takes on just a little color around the edges. Transfer the pan to a rack and cool to room temperature before filling.
Filling: Spread the dulce de leche over the shortbread base and sprinkle over the whole candied nuts or the big pieces.
Topping: Melt the chocolate in a heatproof bowl set over a saucepan of barely simmering water or in a microwave oven, using a low power setting. Remove the chocolate from the heat and gently stir in the butter, stirring until it is fully blended into the chocolate.
Pour the chocolate over the dulce de leche, smoothing it with a long metal icing spatula, then sprinkle over the finely chopped candied peanuts. Slide the pan into the refrigerator to set the topping, about 20 minutes; if you'd like to serve the squares cold, keep them refrigerated for at least 3 hours before cutting.
Cut into bars, each roughly 2 to 2½ inches on a side. Yield: 24.
* To make your own Dulce de Leche , (from David Lebovitz's site ):
Preheat the oven to 425°F. Pour one 14-ounce can of sweetened condensed milk (not evaporated milk) into a glass pie plate or shallow baking dish. Stir in a few flecks of sea salt.
Set the pie plate within a larger pan, such as a roasting pan, and add hot water until it reaches halfway up the side of the pie plate.
Cover the pie plate snugly with aluminum foil and bake for 1 to 1¼ hours. (Check a few times during baking and add more water to the roasting pan as necessary).
Once the Dulce de Leche is nicely browned and caramelized, remove from the oven and let cool. Once cool, whisk until smooth.
Store in the refrigerator until ready to serve. Warm gently in a warm water bath or microwave oven before using.
