In a bowl, mix together flour, water and salt until smooth. Divide the dough into two parts.
Roll out each dough ball into a very thin rectangle. Brush each of the dough rectangles with melted butter. Stack them together and brush with butter again.
Roll up the dough rectangle into a log, keeping it fairly tight as you go. Cover with plastic and put in refrigerator for 1 hour.
Place the Irvine’s Chicken in a bowl, add the onion, coriander and cumin. Season with salt and pepper.
Cut the dough into 12 pieces. Put each dough piece cut side up and gently roll out into a circle about 5cm in diameter.
Place the filling in the centre of each circle. Dip your finger tips in flour. Bring two sides of a circle together, pinch tightly to seal and then pinch the third side to form a pyramid. Make sure all the edges are sealed well.
Preheat oven to 190°C. Place samoosas sealed side down on a baking sheet, previously covered with parchment paper.
Brush samoosas with the egg and sprinkle with black sesame seeds.
Bake in the preheated oven for about 30 minutes, or until golden brown. Serve hot.