Shift the all purpose flour ina deep bowl and add the cubed butter to it.
Rub the butter into the flour with the tip of your fingers till well incorporated and the mixture resembles breadcrumbs.
Add the mustard powder, grated cheese and the Nigella seeds to the flour and knead into a dough.Do not apply too much pressure while kneading.
Cover the dough with a cling film and refrigerate for 1-2 hours.
After 1 hour divide dough into 2 equal halves, wait for 10 minutes for the dough to soften before rolling out.
Roll out the dough into 3-5 mm thick discs, cut out rounds with a fluted cookie cutter,place them on greased and lined cookie trays, sprinkle some sesame and Nigella seeds on them and press gently and chill them for 15 mins in the fridge.
Bake in a preheated oven at 180 degrees C for 15-17 minutes.