Vary the seeds to go with the flavor of burger, such as sesame for tuna or cumin seeds for lamb.
Place 1 1/2 cups warm water (about 110 degrees), dry milk, butter, and sugar in a large mixing bowl and stir to combine.
Sprinkle yeast over mixture, whisk, and let stand until yeast is dissolved and frothy, about 10 minutes. Add egg, 1 yolk, 2 cups of flour, and salt and whisk until blended and smooth. Add 2 1/2 cups flour and stir with a wooden spoon. When the mixture becomes too thick to stir, use your hands to continue mixing. Add up to another 1/2 cup flour to achieve the right consistency: dough should be just tacky when pinched but should not stick to your fingers. Turn out onto a lightly floured work surface; knead for 1 minute. Let dough stand 10 minutes.
Knead until dough is smooth and elastic, about 5 minutes. Place in a lightly oiled bowl, cover with plastic wrap, and leave in a warm place until doubled in bulk, 45 minutes.
Punch down dough and halve. Cut each half into sixths and form flattened balls. Arrange buns 3 inches apart on parchment-lined baking sheets. Cover with plastic; let stand to double in bulk, 45 minutes.
Heat oven to 400 degrees. Whisk together remaining egg yolk and milk; brush lightly over buns. Distribute 1 tablespoon seeds or 2/3 cup caramelized onions over buns. Bake until golden and hollow sounding when tapped on the bottom, 13 to 15 minutes. Cool on wire racks. Use immediately or freeze the same day.