• Add the warm milk to a bowl of a stand mixer or large mixing bowl if you're kneading by hand. Sprinkle yeast and sugar over the milk. 
• If using active dry yeast, let this sit for around 5-10 minutes until it becomes foamy; if using instant yeast, this step can be skipped.
• Add the flour, egg, and salt to the milk mixture. Fit the stand mixer with a dough hook attachment and mix on low speed until a thick dough forms and the gluten begins to develop.
• Add the softened butter and mix on medium speed for about 10 minutes until the dough is smooth and strong and passes the window pane test. This is when you stretch a piece of dough so thin you can almost see through it. 
• You can also do this by hand.  Squish the butter in until it's all combined. It will become a very sticky dough. Tip the dough onto a lightly floured work space and slap and fold the dough for around 10 minutes until smooth and strong.
• Once kneaded, form the dough into a smooth ball. Transfer the dough to a large bowl and cover it with a damp towel.

Shaping

• Deflate the dough and pull it onto a lightly floured work surface. Weigh the dough and divide it into 20 equal pieces with a bench scraper or sharp knife.
• Roll each piece into a tight ball. Line a 9x13-inch (23 x 33cm) baking dish with parchment paper and place in the dough balls. 
• Loosely cover them and let them rise again until they double in size.
• Preheat the oven to 350°F/180°C.
• Bake the dinner rolls for 20-25 minutes until golden brown. 
• Once out of the oven, brush the rolls with melted butter and sprinkle with flaky sea salt. Let them cool to room temperature on a wire rack before serving.