Combine the warmed milk, instant yeast, and sugar in the bowl of a stand mixer fitted with a dough hook*. Add the flour, sourdough discard, beaten egg, chopped herbs, salt, and garlic powder. With the mixer on low speed, slowly add 2 Tablespoons of melted butter. Increase speed to medium and mix until a shaggy dough forms.
Transfer the dough to a smooth work surface and knead until a smooth dough forms, about 3-4 minutes. The dough should be smooth and slightly tacky (but not sticky). If needed, add more flour or milk (1 Tablespoon / 10 grams at a time) to reach your desired consistency.
Transfer the dough to a large greased bowl and cover with plastic wrap. Let rise for 60-90 minutes, or until doubled in size.
While the dough rises, make the topping. In a small bowl, combine the softened butter, chopped herbs, garlic powder, and ½ teaspoon salt. Mix until combined. Set aside 1 Tablespoon of this mixture to brush on the loaf when it comes out of the oven. Grease a 9x5-inch loaf pan and set aside**.
After the rise, turn the dough onto a smooth work surface and divide into 12 equal-sized pieces. I recommend using a kitchen scale to divide the dough evenly (for me, each piece was about 55 grams).Flatten each piece with the palm of your hand into a rough circle about 4 inches in diameter, then spread with 1-2 teaspoons of the butter mixture. Fold the piece in half like a taco, then transfer to the prepared baking pan with the fold-side down on the bottom of the pan. Repeat with the remaining pieces of dough, using all but the 1 Tablespoon of the butter mixture you've kept for brushing on the loaf when it comes out of the oven.
Cover the pan with plastic wrap or a clean kitchen towel and let rise 30-45 minutes until the dough is puffed. The dough should not rise above the edges of the loaf pan.
Preheat the oven to 350℉.
Bake for 30 minutes, then cover with aluminum foil and bake an additional 5-10 minutes. The final loaf will be golden brown, and if you use a kitchen thermometer, the internal temperature will reach about 200℉. While the loaf is still warm, brush with the remaining butter mixture and sprinkle with flaky sea salt. Let the loaf cool in the pan for 10 minutes, then serve.
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Notes
*This recipe can be made without a stand mixer. To do so, mix the dough in a large mixing bowl with a wooden spoon, then knead by hand. **If you are doubling or tripling this recipe, you will use 2 (for doubling) or 3 (for tripling) 9x5-inch baking pans. To make this recipe with active sourdough starter, omit the yeast and replace the sourdough discard with active starter (200g). You will also need to increase the rise times.