So in theory ひやりと is an adverb, which means it should be able to modify either 冷たい or 撫でる.
These are my interpretations of the sentence, considering the possibilities:
1)At the same time, a blood-curdlingly cold hand touched my heart
2)At the same time, a cold hand touched my heart, sending shivers down my spine
So what is it, and how can I tell?