for the frosting, in Step 3 to leave the mixer on high for about 1 hour until the mixture is cool. Is that correct or a misprint?

Hi Julie! Yes, you read it correctly, it's about an hour. You could try wrapping a chilled towel around the mixing bowl or placing bags of ice around it to shorten the time, or make an Italian buttercream, which will take less time: https://food52.com/recipes/41452-italian-buttercream