Place 1/3 cup peanut butter in a microwave-safe bowl and heat on high for one to two minutes or until melted and thin. Add 1/2 cup nonfat dry milk and 1 cup water and stir until blended. Return it to the microwave for another minute or two. Stir until blended and smooth. Add 1/2 cup nonfat plain yogurt and mix well. Spray an icecube tray with non-stick cooking spray. Pour the mixture into tray and freeze. Treat your dogto one or two ice-cube-sized servings on a hot day for a tasty, high-protein snack.