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1
Stir together sugar, warm water, yeast, and instant dry milk in bowl of a heavy-duty stand mixer; let sit 3 minutes.
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2
Stir together salt, baking powder, baking soda, and 1 cup of the flour in a separate bowl. Add flour mixture to yeast mixture, and beat on low with dough hook until incorporated. Stop mixer, and add 1 1/2 cups flour. Beat on low until incorporated. Gradually add melted butter, beating until incorporated. Increase speed to medium, and gradually add remaining 1 1/4 cups flour, beating until dough pulls away from sides of bowl.
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3
Transfer dough to a lightly floured surface, and knead until dough is completely smooth and slightly tacky to the touch but not sticky, about 10 minutes. (Knead in more flour, if necessary.)
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4
Place dough in a lightly greased bowl, and cover with plastic wrap. Let rise in a warm place (80u00b0F to 85u00b0F), free from drafts, until doubled in bulk, about 1 hour.
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5
Punch down dough, and shape into 15 (1 1/2-inch) balls. Flatten balls into 3-inch-long logs. Place logs, 1/2 inch apart, on a parchment paper-lined baking sheet. Cover with plastic wrap, and let rise until doubled in bulk, about 1 hour.
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6
Steam buns, in batches, in a covered steamer basket over boiling water until tender, about 12 minutes. Remove basket, and cool, uncovered, 3 minutes. Refrigerate in an airtight container up to 3 days, or freeze up to 2 weeks.
