• In a small saucepan, heat the milk and butter over low heat until the butter is melted. Let it cool to lukewarm.
• In a large mixing bowl, combine 2 cups of flour, sugar, yeast, and salt.
• Add the lukewarm milk mixture and eggs to the dry ingredients. Stir until well combined.
• Gradually add the remaining 2 cups of flour until a soft dough forms.
• Knead the dough on a floured surface for about 5 minutes until it's smooth and elastic. You may need to add a little more flour if the dough is too sticky.
• Place the dough in a greased bowl, cover it with a clean kitchen towel, and let it rise in a warm place for about 1 hour or until it doubles in size.

Prepare the Filling:

• While the dough is rising, combine the diced apples, brown sugar, granulated sugar, cinnamon, and nutmeg in a mixing bowl.

Assemble the Cinnamon Bun:

• Once the dough has doubled in size, punch it down, and roll it out on a floured surface to a large rectangle, approximately 18x24 inches.
• Brush the melted butter over the entire surface of the dough.
• Spread the apple filling evenly over the dough.

Roll the Dough:

• Carefully roll the dough into a log, starting from one long edge.
• Slice the log into 12 equal-sized rolls.

Bake:

• Place the rolls in a greased 9x13-inch baking dish. Cover with a kitchen towel and let them rise for an additional 30 minutes.
• Preheat your oven to 350°F (175°C).
• Bake the cinnamon bun for 30-35 minutes, or until it's golden brown and the apples are soft.

Make the Glaze:

• While the cinnamon bun is baking, prepare the glaze by mixing the powdered sugar, milk, and vanilla extract in a bowl. Adjust the milk amount to achieve your desired consistency.

Finish and Serve:

• Drizzle the glaze over the warm cinnamon bun as soon as it comes out of the oven.
• Serve the giant apple cinnamon bun while it's still warm and gooey.