Preheat oven to 375°F, grease a 9x13 baking pan and line with parchment paper.

Sift the flour into a bowl.

Grate the butter into the flour and mix well until the mixture resembles breadcrumbs.

Mix in the brown sugar until the dough forms a ball.

Press the dough into the prepared pan.

Prick the dough all over with a fork.

Bake until just set and lightly golden brown, about 17-20 minutes.

Let cool.

Combine the condensed milk, butter and brown sugar in a nonstick saucepan over medium-low heat.

Cook, stirring constantly, until the mixture comes to a boil.

Reduce the heat and cook until golden and pulling away from the sides of the pan, about 12 minutes, stirring often.

Remove from heat and stir in the peanut butter until smooth.

Pour the caramel sauce over the cooled shortbread and refrigerate until set, about 1 hour.

Place the chocolate in a microwave-safe bowl and microwave in 20 second intervals, stirring in between, until melted and smooth.

Pour the chocolate over the cooled caramel and spread into an even thin layer.

Sprinkle with the chopped peanuts and flaky salt and refrigerate until the chocolate hardens, about 15 minutes.

Cut into 40 squares before serving.