Plum Vareniki (Plum Pierogies)

Bahan-bahan (6 porsi)

• 1/3 cup buttermilk
• 1/2 Tbsp sour cream
• 1 cup warm water
• 1 large egg
• 1 tsp salt
• 3 1/2 cups plus 3 Tbsp all-purpose (unbleached flour)
• 1 lb sweet plums (thinly sliced)
• 1/2 cup sugar