1 1/2 cups warm water (110°F) 2/3 cup instant nonfat dry milk 1/3 cup unsalted melted butter, cooled 3 tablespoons sugar 2 1/4-ounce packages active dry yeast (1 tablespoon plus 1 teaspoon) 1 large egg 2 large egg yolks 5 cups all-purpose flour, plus more for dusting 1 1/2 teaspoons kosher salt Canola oil, for bowl 2 tablespoons milk 1 tablespoon seeds, such as sesame, poppy, fennel, or cumin, optional
1. Place the warm water, dry milk, butter, and sugar in a large mixing bowl, and stir to combine. Sprinkle the yeast over the mixture, whisk, and let stand until yeast is foamy, about 10 minutes.
2. Add egg, 1 egg yolk, 2 cups flour, and salt; whisk until smooth. Add 2 1/2 cups flour; stir with a wooden. spoon. When mixture becomes too thick to stir, use your hands. Add up to another 1/2 cup flour until dough is tacky when pinched but not sticky. Turn out onto a lightly floured work surface; knead for 1 minute. Let stand 10 minutes.
3. Knead dough again until smooth and elastic, about 5 minutes. Place in a lightly oiled bowl; cover with plastic wrap. Leave in a warm place until doubled in size, about 45 minutes. Punch down dough and divide in half. Cut each half into sixths; form flattened balls. Arrange buns 3 inches apart on parchment-lined baking sheets. Cover; let stand until doubled in size, 45 minutes.
4. Preheat the oven to 400°F. Whisk remaining egg yolk and milk; brush egg wash lightly over buns. Top buns with the seeds, if using. Bake until golden and hollow sounding when tapped, 13 to 15 minutes. Cool on a wire rack. Buns can be frozen in resealable plastic bags up to 2 months.