Begin by making the water roux. Whisk together Water (125 milliliter) and Bread Flour (25 gram). Whisk until smooth. Cook on medium heat and bring to a gentle boil. Continue to stir until smooth and thickened. Set aside.
Separate the Egg (3) Yolks.
Peel, core and dice the Granny Smith Apple (2). Set aside.
Put 120 grams of the roux, Salt (1 teaspoon), Granulated Sugar (55 gram), Bread Flour (325 gram), Milk Powder (1 tablespoon), Ground Cinnamon (1 1/2 teaspoon), Instant Dry Yeast (1 1/2 teaspoon), and Unsalted Butter (35 gram) onto a clean working surface, and make a well.
Add 125 ml of the Milk (125 milliliter) and the Egg (1) into the well. Knead until it becomes a dough and gluten has developed. Cover with a wet towel until it doubles in size, approximately 45-60 minutes.
Meanwhile, sift the Corn Flour (25 gram) together with Granulated Sugar (50 gram) and mix in the 3 egg yolks until combined.
Heat remaining Milk (300 milliliter) until shimmering. Temper 1 to 2 tablespoons of hot milk with the egg and sugars mixture and eventually add in all of the milk and mix together.
Bring the mixture back to the pot and keep stirring until thickened. Quickly add in the Unsalted Butter (10 gram) and keep stirring until combined. Put in fridge and set aside for use later.
Heat the Unsalted Butter (20 gram) in a pan. Combine diced apples with Lemon Sugar (1 tablespoon), Ground Cinnamon (1 1/2 teaspoon), and Granulated Sugar (2 tablespoon) to prevent oxidizing.
In a pan over medium heat, pan fry the apples until soft and tender.
Combine the cinnamon apple with the custard, mix well together.
Once it has cooled down, put about 1 ½ tablespoons of cinnamon apple custard into cling wrap and form a ball shape. Put in freezer to harden so it easier to fold in later.
Your dough should be done by now. Transfer to clean floured working surface, gently deflate the dough and squeeze all the air out. Divide dough into 12 portions about 60g each.
Roll the dough into balls and let rest for 15 minutes.
Roll the dough flat and fold in the custard.
Put the dough balls into a lined muffin tray. Cover with cling wrap and rest for 45-60 minutes or until doubled in size.
Preheat oven to 180 degrees C (350 degrees F). Beat together an Egg (1) and Milk (2 tablespoon) to make an egg wash. Brush the egg wash on top of each bun.
Bake for about 20 minutes until golden brown.
Transfer the baked buns to a rack and let them cool. Once the buns have cooled down, brush a thin layer of melted Unsalted Butter (20 gram) on top.
To make the cinnamon sugar topping, mix the Ground Cinnamon (1 tablespoon) with the Granulated Sugar (50 gram). Roll the top of each bun in the cinnamon sugar. Serve and enjoy!