In the bowl of a stand mixer, combine warm water (about 120°F) with instant yeast. Let stand 5 minutes, until foamy.
In a separate bowl, whisk bread flour, salt, baking powder, and sugar. Add to the mixer and blend on low speed until combined.
Add the softened butter 1 tablespoon at a time, mixing until fully incorporated.
Knead with the dough hook for 5–7 minutes, until smooth, elastic, and slightly tacky.
Form the dough into a ball, return to the bowl, cover, and let rise in a warm place for 1 hour, or until doubled.
Punch down the dough. Divide into 12 equal portions (about 3 oz each). Roll each into a tight ball.
Arrange dough balls in a greased 9x13-inch baking dish. Cover loosely and let rise for 45 minutes until puffy.
Preheat oven to 400°F (200°C). Whisk the egg with 1 tablespoon water and brush the tops and sides generously.
Bake 15–17 minutes, until golden brown. Serve warm.