Cream the butter and sugar together, then add the eggs, one at a time, beating well after each addition. Meanwhile, sift the flour with the baking powder and salt and add gradually to the batter, beating on medium speed. Combine the yogurt with the baking soda and add to the batter, a few tablespoons at a time, along with the vanilla. Mix together the raisins, spices, and if desired, the walnuts. Pour half the batter into a buttered 23 cm-square pan and sprinkle half the raisin mixture evenly over it. Pour in the rest of the batter and top with the raisin mixture. Bake in a moderate oven (350 degrees F) for 40 to 45 minutes, or until cake springs back to the touch of your finger. Remove to a cake rack and cool in the pan. Cut into squares or diamond shapes when cool. Note: Cupcakes are more popular than cake squares in the larger Greek cities. This recipe would make a dozen cupcakes. *