Place the sugar and butter (softened at room temperature) into a large bowl and beat using an electric hand mixer for about 5 minutes, until fluffy and light in colour.
Add the flour, salt and the sumac and mix together, until it forms a dough.
Place your dough between two sheets of parchment paper and, using a rolling pin, roll out to a 1 cm thickness. Cut the dough into 2cm wide by 9cm long rectangles, and place onto a baking tray lined with parchment paper, making sure to leave space between each biscuit as they will expand a little.
Place your trays in the fridge for 20 minutes or in the freezer for 10 minutes, until the biscuits are firm.
Bring your biscuits out of the fridge or freezer and bake in the preheated oven for 25 to 30 minutes, until lightly golden in colour.
Whilst the biscuits are cooling, place your white chocolate in a bowl, and melt in the microwave for 30 seconds until it has partially melted. Vigorously mix the chocolate until it is fully melted. If little lumps of chocolate remain, place it for a few extra seconds in the microwave.
Dip a third of each shortbread biscuits into the white chocolate and finish by sprinkling a little sumac over the chocolate.