Prepare the Dough: In a bowl, dissolve yeast and sugar in warm milk. Let sit for 5-10 minutes until foamy. Gradually add flour and salt, kneading until a smooth dough forms. Cover and let rise for 1 hour until doubled in size.
Make the Filling: While the dough rises, mash the boiled potatoes and mix with cheddar cheese, melted butter, black pepper, salt, and garlic powder.
Assemble the Pie: Preheat oven to 180°C (350°F). Roll out half the dough and place it in a greased baking dish. Spread the potato-cheese filling evenly over the dough. Roll out the remaining dough and place it on top, sealing the edges. Brush the top with melted butter or an egg wash.
Bake & Serve: Bake for 30-35 minutes or until golden brown. Let cool for 10 minutes before slicing. Serve warm and enjoy!