With the rack in the middle position, preheat the oven to 350°F (180°C). Butter a 17 x 12-inch (43 x 30 cm) baking sheet.
In a bowl, combine the flour and sugar. Mix in the butter with a wooden spoon. Using your fingers, presse the dough into the bottom of the baking dish.
Bake for 17 minutes or until the crust is slightly golden. Set aside.
In a pot over medium-high heat, cook the sugar, water and corn syrup without stirring until it forms a light caramel. Off the heat, add the condensed milk, butter and salt. Mix well with a wooden spoon. Watch out for splattering. Stir in the peanuts and mix well to combine. With a spatula, spread the caramel evenly over the crust.
Bake for 15 minutes. Let cool completely on a wire rack, about 1 hour.
With a spoon, splatter the melted chocolate over the surface of the caramel. Let set at room temperature for 1 hour. Cut into squares and serve. The squares will keep in an airtight container for 1 week at room temperature.