Lightly spoon flours into dry measuring cups, and level with a knife. Combine 1 1/2 cups bread flour, whole-wheat flour, granulated sugar, salt, and yeast in a large bowl. Add water, potatoes, and oil; stir until a soft dough forms.
Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 8 minutes); add enough of remaining bread flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 45 minutes or until doubled in size. (Press two fingers into the dough. If indentation remains, the dough has risen enough.) Punch dough down. Turn dough out onto a lightly floured surface; roll into a 20 x 7-inch rectangle. Brush butter over dough, leaving a 1/2-inch border; sprinkle with 1/4 cup brown sugar and cinnamon. Beginning with a long side, roll up jelly-roll fashion; pinch seam to seal (do not seal ends).
Coat a 13 x 9-inch baking dish with cooking spray; sprinkle pecans and 1/2 cup brown sugar in bottom of dish.
Place a long piece of dental floss or string under dough 1 inch from end of roll. Cross ends of string over top of roll; slowly pull the ends to cut through the dough. Place roll portion, cut side up, in prepared pan. Repeat procedure with remaining dough. Carefully pour the milk around roll portions. Cover and let rise 20 minutes or until doubled in size.
Preheat oven to 375°.
Uncover dough; bake at 375° for 20 minutes or until lightly browned. Cool in dish 5 minutes on a wire rack. Place a serving platter upside down on top of rolls; invert onto platter.