Why are my beignets hollow?
I had the same problem too, and thought that I was going crazy because I’ve tried so many different recipes and always end up with hollow beignets!   

What I eventually found out was that when I fried my dough in lower heat, the longer time allowed the dough to expand more, which resulted in bigger, puffy, hollow beignets.  

When you cook them in medium heat, the quicker frying time allows the heat to expand just the outside and a little bit in the center- this is what makes it less hollow and more pillowy.