In a small bowl, combine warm milk and water with yeast, and 1 tablespoon sugar. Let stand until foamy, about 5 minutes.
In a larger bowl, whisk together flour and salt.
In the bowl of your stand mixer, fitted with the paddle attachment, beat butter and remaining 2 tablespoons sugar on medium speed until light and fluffy, about 2-3 minutes. Add in egg, the bloomed yeast, and one cup of your flour mixture and mix until well combined, about a minute. Add the rest of the flour and beat on medium speed until a soft, smooth, slightly sticky dough forms, about 2 minutes.
Switching to the dough hook, increase speed to medium-high and knead for 3 minutes.
Lightly oil a large bowl or coat with nonstick spray; place dough in bowl, turning to coat. Cover and allow to rise in a warm spot 1 to 2 hours, or until doubled in size.
Gently deflate dough by punching down. Divide the dough into 12 equal pieces. Press each piece into a disk, fold all the edges in towards the center of the disk. Turn seam side down and shape into a ball by rolling on the counter in your cupped hand.
Spray a 13X9" pan with cooking spray. Place your rolled balls into the pan, in three rows of four. Cover again and leave in a warm place to double again, about 1 hour.
While the rolls are rising, preheat the oven to 400°. Brush the tops of your rolls with a egg wash and bake for 20-25 minutes.
Brush finished rolls with melted butter and sprinkle with flaky sea salt if desired.