When rolling out a wrapper for bao, look at the center of the dough disk and imagine a quarter-sized circle. This is what the Chinese call the belly of the wrapper. You want to roll out the disk into a flat, round wrapper that retains a thick belly. A lightweight Asian-style rolling pin, which is essentially a three-fourths-inch wooden dowel, is ideal for this job.
With this in mind, start rolling out the disk from the center to the rim. Apply more pressure to the outer one-half- to three-fourths-inch border of the wrapper. If you are using the skinny Asian rolling pin, work it with one hand in short downward motions while your other hand rotates the dough disk. Regardless of the tool used, be sure to frequently lift and rotate the dough as you work to ensure an even shape and make sure the wrapper doesn't stick to your work surface. Aim for the wrapper size indicated in the recipe. Set aside the finished wrapper while you roll out more.
-- Andrea Nguyen