I'm following this recipe: http://blog.junbelen.com/2010/03/24/how-to-make-pan-de-sal-filipino-bread-rolls-at-home/. I was careful to measure the water temperature for the yeast, etc. The resulting bread rolls came out a bit dense.  I used flour that was 11.5 grams protein per 100 grams. I used a mixer for about 12 minutes until I got a good window pane (though it was still breaking apart a bit, I didn't want to overmix so I stopped).  What can I do to make the buns less dense? Should I use a different type of flour? Mix more? Add baking powder (if so, how much???)? 
There should have been 3 rises. The first till it doubled, then an hour after molding it, and finally an hour after cutting the rolls. The dough should have gotten noticeably bigger during each rise, and if this is the case, there's really no way for them to be dense.  If it doubled in the first rise but didn't seem to puff up much by the last rise, your yeast may have simply run out of fuel. To combat this, you can either add a sugar to the dough so the yeast has more food, or cut the rising times a bit so that there's more food for the yeast left in the dough by the last rise.  When punching the dough down, be fairly gentle. You want to knock out the excess gas, but you should be leaving some in. For instance, if the dough doubled in size, you'd want to punch it down to about 1.25 to 1.5 times the original size, not all the way down.  I wouldn't add baking powder to these. Most of the rising potential of the baking powder will be spent long before they're ready to go in the oven. Even if you use a double acting one, it will dramatically change the character of the rolls leaving you with a totally different end result. 