Cheesy Potato Pierogies

Ingredients

• 1 tablespoon water
• 1/2 pound Yukon Gold potatoes, peeled and diced
• 1 large egg
• 1/2 teaspoon salt
• 1 tablespoon whole milk
• 1/4 cup shredded Cheddar cheese
• 1/2 cup sour cream
• 2 cups all-purpose flour
• 3 tablespoons unsalted butter, melted