1. Preheat oven to 350ºF. Butter an 8″ square pan and set aside.
2. Pulse the flour, sugar, confectioners’ sugar and salt in a food processor a few times. Then, add in the cold butter and pulse about a dozen times until the mixture looks like a coarse meal. Pour the yolk in, pulse just until a ball of dough forms. Press this ball into the bottom of the cake pan. Prick it all over with the tines of a fork and bake for 15-20 minutes or just until the edges begin to color.
3. To make the filling: line a baking sheet with parchment paper or silpat. In a medium saucepan put the sugar and water in the saucepan and cook it over medium heat until the sugar dissolves. Then, continue cooking, without stirring just until it starts to turn an amber brown. Toss in the peanuts and start stirring with a LONG handled spoon. The peanuts will get coated with sugar and turn white, then turn a nice amber caramel. Remove them from the pan when they are lightly browned, but not burned, and turn them onto the lined back sheet to cool.
4. When the crust is cool layer on the dulce de leche and sprinkler top of it with half of the carmelized peanuts.
5. Melt the chocolate and butter in the microwave, or in a heatproof bowl set over simmering water, until just smooth and combined. Pour this over the dulce de leche.
6. Chop the remaining peanuts finely and sprinkle them over the chocolate.
7. Cool the entire pan in the refrigerator for at least 3 hours if you’d like to serve them cold. They can stay, covered, in the fridge for a few days.