Beat egg whites in a large bowl until stiff but not dry. In separate bowl, stir together sugar, flour and salt. Whisk in lemon zest, lemon juice, butter or margarine, milk, yogurt and egg yolks. Gently fold whipped whites into lemon mixture. Pour into greased 1 quart (1 L) baking dish. Place baking dish in a shallow pan of hot water so that the water rises 1 inch (2.5 cm) up the sides of the baing dish. Bake in a preheated 180°C (350°F) oven until top is lightly browned, about 35 to 40 minutes. Serve warm or cold.
Tip: Sprinkle pudding cake with icing sugar or top with a dollop of whipped cream.