Combine water, yeast and sugar in large bowl of electric mixer fitted with paddle attachment. Mix until sugar and yeast dissolve. Add milk, 3/4 cup bread flour and 3/4 cup all purpose flour and mix until smooth. Set aside at room temperature for 30 minutes.
Mix remaining 21/2 cups bread flour and 21/2 cups all purpose flour in medium bowl. Remove 1 cup flour mixture and transfer to small bowl. Add 1 tablespoon salt and baking powder to flour in small bowl and stir with fork to blend. Add this flour mixture to sponge and mix to blend. Continue mixing, gradually adding enough of remaining flour mixture until dough leaves side of bowl. Change attachment on mixer to dough hook. Knead with mixer until dough is moderately firm and dry, adding more of flour mixture as necessary. Knead 10 to 15 minutes longer. Lightly oil large bowl. Add dough; turn to coat. Cover with damp towel. Let stand in warm draftfree area until doubled in volume, about 1 hour.
Punch down dough. Transfer to lightly floured work surface. Knead briefly. Divide dough into 9 equal portions. Roll each into ball shape. Cover with towel and let rest 10 minutes.
Combine butter and remaining 1 tablespoon flour in small bowl. Pull each dough ball into 6 inch long log. Using sharp knife, cut shallow slit down center of each log. Spread some of butter mixture inside each. Roll logs briefly on counter to enclose butter mixture. Arrange on large parchment lined baking sheet. Cover with towel. Let stand in warm draftfree area 30 minutes. Rising is complete when finger pressed in dough leaves a mark.
Preheat oven to 325 degrees F. Combine remaining 1 tablespoon salt and 1 cup water in spray bottle. Generously spray rolls all over. Bake 5 minutes. Reduce oven temperature to 325 degrees F. Bake rolls until golden brown and sound hollow when tapped on bottom, about 10 minutes. Transfer to racks and cool completely.
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