In a large mixing bowl, combine the milk, sugar, egg, and yeast. Mix until well combined. Add the flour and salt, and mix until no dry flour remains.
Cover the dough and let it rest in the fridge for 30 minutes.
After 30 minutes, take the dough out and perform the first stretch and fold. Pull one side of the dough and fold it toward the center. Repeat for all four sides.
Turn the dough upside down, cover, and return it to the fridge for another 30 minutes.
Remove the dough and add the butter. Knead until the butter is fully incorporated. Cover and let it proof at room temperature for about 1 hour, or until doubled in size.*(If using a stand mixer, you can skip the resting steps—mix until smooth, then add the butter at the end.)*
Meanwhile, prepare the potato filling by combining the mashed potatoes, mozzarella cheese, mayonnaise, salt, and pepper. Mix well.
Divide the filling into 8 portions and shape each into an oval.
Place the filling in the fridge for 30 minutes to firm up, making it easier to handle.
Punch down the dough to release the air. Divide it into 8 equal portions, roll into balls, cover, and let rest for 20 minutes.
Roll each dough ball into an oval shape. Place the potato filling in the center, then seal and pinch the edges together.
Mix the cocoa powder and bread flour together.
Coat each dough piece with the cocoa mixture and dust off any excess. Use a chopstick to poke small holes to create “potato eyes.” Be careful not to poke all the way through, or the filling may leak during baking.
Place the dough pieces on a baking sheet, cover, and let them rest for another 40 minutes.
Preheat the oven to 175°C. Bake for 25 minutes. After 10 minutes, cover the bread loosely with foil to prevent over-browning.
Let them cool on a wire rack before serving. Enjoy!
