Melt butter and mix well with the crumbled shortbread cookies. Press the mixture into the bottom of a square baking pan (23x23cm).
Sprinkle the chopped pistachios over the shortbread-butter mixture. Sprinkle the grated chocolate and then the nuts over the top. Finally, pour Bonny Sweetened Condensed Milk evenly over the mixture (do not mix – just pour).
Bake in the pre-heated oven at 180°C for about 30 minutes. Allow to cool on a cake rack and then cut into squares of equal size. Dip halfway in the melted chocolate and allow to cool on butter paper.