Start by making a tangzhong. To a small pot, add ¼ cup of your milk and 2 tablespoons of your flour. Place on the stove over medium-low heat and whisk constantly until a thick paste forms. Remove from heat and set aside
Warm your remaining ½ cup of milk in the microwave for about 20-30 seconds. It should be warm when you touch it with your finger but not hot. If it’s too hot, you might kill the yeast!
Pour your warmed milk into the bowl of your mixer and sprinkle your yeast over the top. Give it a light stir and allow it to sit for 5 minutes to activate your yeast and get a little frothy
Add in your tangzhong, sugar, sweet potato, melted butter, vanilla, salt, cinnamon, and remaining 3 cups plus 2 tablespoons of flour. Fit your mixer with the dough hook attachment and mix on low speed until a shaggy dough forms
Scrap down the sides and bottom of your bowl, then increase your mixer to a medium speed for 5-10 minutes until your dough is smooth and elastic. To check if your dough is ready, pull off a piece of dough and gently pull it very thin. You should be able to hold it up and see light through the dough without it tearing
Lightly grease a large bowl and place your dough inside. Cover with a clean towel and place in a warm spot to rise for 60-90 minutes, or until doubled in size. If your kitchen is warm, it might rise faster, if cooler, it might rise slower. I highly recommend taking a picture of your dough before covering it to compare!
While your dough is rising, make your cinnamon-sugar filling. Simply mix together your brown sugar and cinnamon in a bowl, set aside, and place your softened butter in another small bowl - you want it spreadable but not melted
Grease and line a 9”x5” loaf pan with parchment paper
When your dough is ready to shape, punch it down to release the air, turn it onto a lightly floured surface, and roll it to a 12”x16” rectangle
Spread your softened butter evenly over the dough before sprinkling over your cinnamon sugar. Use your fingers to gently rub your cinnamon sugar into the butter, to help it stick and form a sort of paste
Using a pizza cutter or sharp knife, slice your dough vertically into three equal strips and then horizontally into 5 equal strips. You should have 15 squares of dough
Dough side down (cinnamon-sugar side up) stack your dough so the bottoms match up. Then carefully place (bottom edge down) in your loaf pan. They should fit fairly snuggly, with a little room to grow
Cover with your clean towel and let the dough rise in a warm sport for another 30-45 minutes, until it has puffed and filled the pan
Preheat your oven to 350°F
In a bowl whisk together your egg and a splash of water or milk, to make an egg wash. Brush over the top of your dough, to get that beautiful golden brown bake
Bake for 30-45 minutes on the middle rack, removing from the oven at the 15-minute mark and covering with foil to prevent the top of your loaf from burning. Insert a digital thermometer into the middle of your dough; once it reads 192°F - 195°F, remove it from the oven
Allow the bread to cool in the pan for 10 minutes before removing and placing on a wire rack to cool for another 20 minutes
To make your glaze, add your melted butter to a bowl and sift in your powdered sugar. Then add in your salt, vanilla, and milk, and whisk until a smooth glaze forms
Drizzle your glaze over the top of your warm bread and leave to set for 15 minutes. Enjoy!