Place the yeast and sugar in the bowl of a stand mixer fitted with a dough hook or paddle attachment. Or, if you do not own a stand mixer, a regular large mixing bowl. Heat the milk on the stove or in the microwave until warm to touch, about 110°F (43°C). Pour warm milk on top of yeast/sugar. Whisk gently to combine, then loosely cover with a clean kitchen towel and allow to sit for 5-10 minutes. The mixture will be frothy after 5-10 minutes.
Add the butter, egg, flour, salt, garlic powder, and rosemary. Beat on low speed for 3 minutes. Dough will be soft.
Keep the dough in the mixer (and switch to the dough hook if using the paddle) and beat for an additional 5 full minutes, or knead by hand on a lightly floured surface for 5 full minutes. (If you’re new to bread-baking, my How to Knead Dough video tutorial can help here.) If the dough becomes too sticky during the kneading process, sprinkle 1 teaspoon of flour at a time on the dough or on the work surface/in the bowl to make a soft, slightly tacky dough. Do not add more flour than you need because you do not want a dry dough. After kneading, the dough should still feel a little soft. Poke it with your finger—if it slowly bounces back, your dough is ready to rise. You can also do a “windowpane test” to see if your dough has been kneaded long enough: tear off a small (roughly golfball-size) piece of dough and gently stretch it out until it’s thin enough for light to pass through it. Hold it up to a window or light. Does light pass through the stretched dough without the dough tearing first? If so, your dough has been kneaded long enough and is ready to rise. If not, keep kneading until it passes the windowpane test.
Shape the kneaded dough into a ball. Place the dough in a greased bowl (I use nonstick spray to grease) and cover with plastic wrap or aluminum foil. Place in a slightly warm environment to rise until doubled in size, around 60-90 minutes. (If desired, use my warm oven trick for rising. See my answer to Where Should Dough Rise? in my Baking with Yeast Guide.)
As the dough rises, prepare the filling in the next step and grease a 9×5-inch loaf pan.
In a medium bowl, mix the soft butter, rosemary, parsley, garlic, and salt together. If the butter is soft enough, you can just mix it all together with a spoon or fork. You can use an electric mixer if that’s easier too. Cover tightly and set aside until ready to use. (Don’t refrigerate unless making well in advance. It’s easiest to spread on the dough when at room temperature. If refrigerated, let it come to room temperature before spreading on dough pieces.)
Punch down the dough to release the air. Place dough on a lightly floured work surface. Divide it into 12 equal pieces, each about ¼ cup of dough and a little larger than a golf ball. Using lightly floured hands, flatten each into a circle that’s about 4 inches in diameter. The circle doesn’t have to be perfectly round. I do not use a rolling pan to flatten, but you certainly can if you want. Spread 1-2 teaspoons of filling mixture onto each. Sprinkle each with 1 Tablespoon of cheese. Fold circles in half and line in prepared baking pan, round side up. See photos above for a visual.
Cover with plastic wrap or aluminum foil and allow to rise once again in a slightly warm environment until puffy, about 45 minutes.
Adjust the oven rack to the lower third position then preheat oven to 350°F (177°C).
Bake until golden brown, about 50 minutes. If you find the top of the loaf is browning too quickly, tent with aluminum foil. (Don’t be alarmed if there’s melted butter around the sides of the bread as it bakes, it will seep into the bread before it finishes.) Remove from the oven and place the pan on a wire rack. If desired, brush with melted butter for topping and sprinkle with sea salt.
Cool for 10 minutes in the pan, then remove from the pan and serve warm.
Cover and store leftovers at room temperature for up to 2 days or in the refrigerator for up to 1 week. Since the bread is extra crispy on the exterior, it will become a little hard after day 1. Reheat in a 300°F (149°C) oven for 10-15 minutes until interior is soft again or warm in the microwave.