Whisk egg, sugar, and salt in a bowl. Gradually add in milk as you are whisking. Sift in all-purpose flour and cornstarch into the mixture and whisk again. There will be some lumps. Don't worry. Put the saucepan over low heat. Add butter and continue to stir until butter started to melt and continue to cook until the custard thickened. Remove from the heat. Cover with a cling wrap touching the custard to prevent a layer of skin from forming. Chill in the fridge until it's firm, about 2 hours or longer
Prepare the dough:
Peel the sweet potato and cut it into small cubes. Steam for 10-15 minutes or until you can easily mash with a fork. Alternatively, you can also pressure cook on high pressure in Instant Pot for 2 minutes if you cut into small cubes. Mash the sweet potoates while they are still warm
Add all ingredients into a mixing bowl. I use a stand mixer fitter with a dough hook attachment. Knead on lowest speed for about 1 minute and then turn one level up and knead. The amount of liquid needed really depends on how moist the sweet potato is. If the dough can't come together because it's too dry, add milk or water, about 1 tablespoon at a time, until you can get a smooth dough. Knead until the dough is soft and not sticky, about 5 minutes or so
First proofing:
Oil a large bowl and put the dough in. Cover and place it in a warm place. It may take 1 to 1 1/2 hour to proof the dough until they are about double in size
Shaping:
Line your bread pan with parchment paper, overlapping on all four sides and let the extra hang out to the side for easier lifting out from the pan later. When you press on the dough, the indentation should stay. The dough is done proofing for the first round. Punch the dough down to release all the air bubbles. Give it a few knead. Cover and let it rest for about 5 minutes. It will be easier to roll out
After that, roll the dough out into about 1/2 inch thickness. The width should be about the same as the length of your bread pan and about 13 inch long. Use a pizza cutter or a dough scraper and cut the other 2/3of the dough into long strips (as shown in the photo above). Make sure you really cut through each strip for the best result. Spread the custard cream on the other 1/3 of the dough
Second proofing:
Fold the two sides over to cover the custard cream and then gently roll up like a Swiss roll. Gently pick it up and place inside the prepared pan. Cover with a towel or plastic wrap and let the dough proof again for the second time until the dough comes up about 1/2 inch above the rim of the pan, may take 45 minutes to 1 hour
Baking:
15 minutes before the end of second round of proofing, preheat oven to 350 F (180 C). Brush the top of the bread with some milk and put the bread pan in the middle of the oven and bake for 30-35 minutes or until lightly golden brown on the top. The bottom of the bread should sound hollow too when you tap on it
Cool down:
Lift the bread up by grabbing the overhanging parchment paper and let the bread cools down completely on a cooling rack before slicing