Combine the milk and water in a microwave safe cup or bowl. Microwave until it reaches about 108-113F. Add yeast and a tsp of sugar and mix. Let it sit for 10 mins to bloom.
To the bowl of a stand mixer, add the bread flour, salt and egg.
Than add in the bloomed yeast. With the dough hook attachment mix on a medium speed until the dough comes together.
Slow the mixer down to low and add 1 tbsp at a time of butter and wait for it to be incorporated into the dough before you add the next.
Once all the butter has been incorporated, let the dough mix on a medium speed for 5 mins or until it releases from the sides of the bowl.
Cover and let it rise for 1-2 hours or until it doubles in size. Once doubled, punch it down and cut into 8 equal pieces.
Form each bun and place onto a parchment lined baking sheet about 4 inches apart.
Cover the buns with a damp kitchen towel and let them rise again for 30-40 mins until nice and puffy. In the meantime, preheat the oven to 350℉.
Once nice and puffy, make the egg wash by combining 1 egg and 1 tbsp of water and whisking those together. Brush each bun with egg wash and sprinkle with seeds, if using.
Bake at 350℉ for 30-40 mins or until golden brown in color.
If you would like to soften the shells, allow the buns to completely cool and place them into a plastic bag or ziploc.