Combine flour, salt and sugar in a big bowl and mix.
Add the cold cubed butters to the flour mixture. Rub them gently with your fingertips until the mixture looks like breadcrumbs.
Add in the egg and gently bring the dough together with a spatula. Don't overwork the dough not to activate the gluten-flour protein.
Divide the dough in half and make balls. Cover each dough with plastic film. Flatten the doughs and let them rest in the fridge for almost 20 minutes. When you divide and flatten the doughs, they rest fast.
Remove the doughs from the fridge. Preheat the oven to 350°F(177C°). Line baking sheet with parchment paper or silicone baking mat. Set aside.
Lightly flour the worksurface and the rolling pin, so the dough won't stick to either. Dust flour over the dough. Roll out the dough almost 8,5*7 inch(22*18cm)long, ⅕ inch(5mm)thick. Use 1,5inch (4cm) cookie cutter. Roll out the dough with the leftovers again. Repeat the same steps with the other half of the dough.
Prepare the egg wash by mixing egg yolk and 1 teaspoon water. Glaze the cookies with egg wash. Sprinkle sesame and nigella seeds on top.
Bake for 10-11 minutes until they have a nice shine and firm. Let them cool for 5 minutes on the baking sheet before transferring them to a wire rack to completely cool.