To Make Dough:Sift flour and salt together into a mixing bowl. Add the melted butter and yogurt and stir with a wooden spoon until the dough resembles a coarse, crumbly meal. Add cold water by the TBSP-full until the dough can be gathered into a ball. Knead on a lightly floured surface for 5-10 mins, until the dough is smooth and elastic. Shape into a ball and sit in a loosely covered bowl while you prepare the filling. To Prepare Filling: Cook potatoes and carrot in water until soft. Drain well, and set aside. Melt 2 TBS butter in a skillet, and the onion, garlic, and ginger and saute until soft. Add salt, coriander, cumin, tumeric,and cayenne, and cook, stirring, until mixture forms a paste and continue cooking for 2 mins. Add the drained vegs, and cook over med high heat, stirring constantly, for 5 mins. Carefully stir in thawed peas, add water and lemon juice, reduce heat to low, cover and cook another 5 mins. Remove filling from heat, and cool. To shape the samosas: Pinch off enough dough to make a ball 1 inch in diameter. On lightly floured surface, roll into a 31/2-4" circle. Cut the circle in half, moisten one of the edges and fold in over on itself and press to seal, making a cone shape. Fill the cone with vegetable mix. Press to seal. OR...cut into squares, fill, and fold into triangles. Heat oil in deep fryer to 370 degrees and fry samosas, removing them as soon as they are brown and crispy. Drain on paper towelling, and keep warm in the oven until you have fried them all. Filled samosas can be refrigerated in an air-tight container up to 3 hours before frying, but not overnight. Serve as an appetizer, with mango chutney as a side. NB. I have used a Puff Pastry, or Phyllo dough, and baked them, and they are just as wonderful. Less calories too, and less fat.