Add flour, sugar, condensed milk, milk, water, egg and yeast into the stand mixer and knead on low speed until the dough is formed.
Cover the mixing bowl and let the dough rest for 30 minutes. Store it in the fridge if your room temperature is higher than 80°F
Meanwhile, bring the butter out from the fridge, cut it into 4-6 pieces and leave it on the counter to soften a little.
When it's done resting, add salt and continue to knead the dough for 1 minute.
Add butter cubes one by one and knead until the dough is very smooth and passes the windowpane test. (This step takes me about 7 minutes, with 6 minutes on low speed and 1 minute on medium speed. The actual time it takes depends on your mixer. It can take up to 15 minutes.)
Form the dough into a ball (it will be very sticky, but if it's too sticky to handle, then add a little bread flour and knead for another 1~2 minutes) and put it in a lightly greased bowl. Cover, and let it rise for 1 hour to 1.5 hour in a warm place (between 80°F to 90°F) until it doubles in size.
Transfer the dough onto a working surface and cut it into 12 even pieces. Cover and let them rest for 5 minutes.
Form each piece into a ball (see the instruction in the main post) and place them in a lined baking pan, cover with greased plastic wrap and then cover with a damp cloth and let it rise in a warm place (between 80°F to 90°F) until it doubles in size (about an hour).
In the last 20 minutes of proofing, preheat the oven to 375°F (190°C).
Preheat the oven to 375°F. Bake at 375°F for 16 minutes or until golden brown.
Brush the top with melted butter and transfer the rolls onto a cooling rack. Let it cool for 10 minutes before enjoying.