1. Prepare the Dough: In a bowl, combine yeast, sugar, and warm milk. Let it sit for 5–10 minutes until foamy. Add flour and salt, then knead until a soft dough forms. Cover and let rise for 1 hour, or until doubled in size.
2. Make the Filling: Mash the boiled potatoes while still warm. Mix in shredded cheese, butter, black pepper, salt, and garlic powder (if using). Stir until smooth and creamy.
3. Assemble the Pie: Preheat oven to 375°F (190°C). Divide the dough into two equal portions. Roll out one half and place it in a greased baking dish. Spread the potato-cheese filling evenly over the dough. Roll out the second dough portion and place it over the filling. Seal the edges and brush with the egg yolk and milk mixture.
4. Bake to Perfection: Bake for 30–35 minutes or until golden brown. Let it cool slightly before slicing and serving.