To make the tangzhong: Combine all of the starter ingredients in a small saucepan, and whisk until no lumps remain., Place the saucepan over medium heat, and cook the mixture, whisking constantly, until thick and the whisk leaves lines on the bottom of the pan. This will probably take only a minute or so. Remove from the heat, and set it aside for several minutes., To make the dough: Mix the tangzhong with the remaining dough ingredients until everything comes together. Let the dough rest, covered, for 20 minutes; this will give the flour a chance to absorb the liquid, making it easier to knead., After 20 minutes, knead the dough — by hand, mixer, or bread machine — to make a smooth, elastic, somewhat sticky dough., Shape the dough into a ball, and let it rest in a lightly greased covered bowl for 60 to 90 minutes, until puffy but not necessarily doubled in bulk., To make the filling: Combine the brown sugar and cinnamon, mixing until the cinnamon is thoroughly distributed., Gently deflate the risen dough, divide it in half, and shape each piece into a rough rectangle., Working with one piece at a time, roll the dough into an 18" x 8" rectangle., Sprinkle half the filling onto the rolled-out dough., Starting with a long edge, roll the dough into a log. With the seam underneath, cut the log into 12 slices, 1 1/2" each., Repeat with the second piece of dough and the remaining filling., Lightly grease a 9" x 13" pan. Space the rolls in the pan., Cover the pan and let the rolls rise for 45 to 60 minutes, until they're crowding one another and are quite puffy., While the rolls are rising, preheat the oven to 350°F with a rack in the bottom third., Uncover the rolls, and bake them for 22 to 25 minutes, until they feel set. They might be just barely browned; that's OK. It's better to under-bake these rolls than bake them too long. Their interior temperature at the center should be about 188°F., While the rolls are baking, stir together the icing ingredients, adding enough of the milk to make a thick spreadable icing. The icing should be quite stiff, about the consistency of softened cream cheese., Remove the rolls from the oven, and turn them out of the pan onto a rack. Spread them with the icing; it'll partially melt into the rolls., Serve the rolls warm. Store completely cool rolls for a couple of days at room temperature, or freeze for up to 1 month.