Sift the flour and baking powder in a large bowl. Add the butter to the flour and gently work with your hands until it resembles a fine bread crumbs.
Now add the egg yolk and bring the dough together. If it looks too crumbly add 1 tbsp of cold water to bring the dough together. Wrap the dough in a cling film and keep it in the refrigerator for minimum of 2 hours.
Don't work too much on the dough it might harden the pastry.
Prepare the filling:
Heat oil in a pan, add the cumin seeds. Then finely chopped garlic and onion. When the onion becomes transparent, add all the spices and stir well.
Finally add the mashed potatoes and mix well. Switch off the flame and let it cool down for a while.
Prepare the pinwheels:
Take the dough out from the refrigerator, roll it out between two sheets of cling film. Spread the filling on the pastry. Roll it like a Swiss roll. Wrap the pastry in a cling film and keep it in the refrigerator for another 30 minutes.
Preheat the oven to 200 degree Celsius. Take the pastry out, cut it into 1 inch pinwheels. Brush the top with beaten egg white and sprinkle with sesame seeds. Place it in a baking tray lined with parchment paper.
Bake it in the preheated oven for 20 to 25 minutes or until golden brown on top.