Combine water and milk powder and stir until poweder is dissolved. Add buttermilk and stir until blended. Let stand at room temperature until thickened. Stir until smooth and refrigerate. Stir before using. Save 1/2 c buttermilk as a base for your next batch. NOTE...I usually leave the buttermilk sit overnight in the summer. If your house is 65 degrees or under it may take about 18 hours to thicken.