Add the milk, instant yeast, unsalted butter and egg to the bowl of Kitchen Aid Deluxe 4.5 Quart Stand Mixer, follow by sugar, bread flour, all-purpose flour and salt.
Using the dough hook attachment, mix all the ingredients together until it forms a dough. Turn the speed to 4 and knead for about 10 minutes.
Once your dough has become smooth, stop kneading and let it proof in a large, greased bowl for 1 hour. After an hour, transfer the dough to a floured surface. Knock out all the air in the dough. Roll the dough to about 16-inch in round shape.
Use a pizza cutter to cut the dough into 16 triangles. Cut 1/2 stick of cold butter into 16 small cubes. Preheat the oven to 350°F (176°C).
Add a butter cube to the bottom part of the dough triangle.
Tug and roll the dough up to form a nice crescent roll shape.
Pinch the end to make sure it is secured. Transfer each butter roll to a baking sheet lined with parchment paper, with the top side facing down. Let it proof for the second time for 1 1/2 hours.
Gently brush the butter rolls with the melted butter. Sprinkle the top with the sea salt flakes and bake in the oven for 10 to 15 minutes, or until the surface of the butter rolls turn light golden brown.
Remove from the oven, serve warm with butter or any fruit preserves/jam.