http://www.pcgamer.com/cuphead-sales/

The old-fashioned platformer Cuphead is pretty great: "A nails-hard 2D shooter where you fight outlandish bosses, interspersed with palate-cleansing run-and-gun sections," as we put it in our very positive review. That high degree of difficulty is well known, and but it doesn't seem to be overly off-putting.

"Cuphead has only been out for two weeks and we never could have dreamed of the reception we’ve received from our amazing fans! Today, we wanted to take a moment to thank each and every one of you for your support and we’re excited to share that Cuphead has gone platinum with over 1 million units sold!" Chad and Jared Moldenhauer of Studio MDHR said. Although the platforms weren't specified, I'm confident they mean the sum across Xbox and PC.