Can't get my peanut brittle out of the pan

I made peanut brittle for the first time yesterday. Although it tasted great, had aesome color and turned nice and hard with no stickiness I couldn't get it off the cookie sheet. I greased it generously with margarine (although the recipe called for butter) but had to eventually pass it over the flame on my gas stove to soften it a bit to remove it. The recipe called to cook it to 305 degrees. Mine may have gone to 310. I'm making this for a baby shower and would live some advise. Thank