I missed this comment! Even though it's 11 days old, I'm hoping you'll see this in your replies box and get my answer anyway: I'm not sure what the popcorn stitch qualifies as, but it's probably some type of border finish since it's so different than the standard granny square motif. It's worked in two rows, with the first row setting the base for the cluster stitches that go into the second row and then the squares are stitched together. The basic pattern is sc in first granny square outside edge space, ch 3, sc in next space, repeat to corner, (sc, ch 3, sc) in the corners, repeat all the way around and join to first sc with a slip st; ch 1 and turn. (Sc, ch 1, sc) in first ch 3 space, yo and insert hook in front loop of first foundation sc, pull through to make three loops on hook, yo and repeat until you have 5 loops on the hook and pull through all five at once for a popcorn stitch. (Sc, ch 1, sc) in next ch 3 space and do a popcorn stitch in every base sc. (Sc, ch 3, sc) in corners. Join with slip stitch to first sc of popcorn stitch row.

And hopefully that makes sense as I'm describing it from memory instead of the actual pattern!