For four season, fans of the HBO show Girls have watched Allison Williams portray Marnie Michaels — the professional counterpart to Lena Dunham's aloof Hannah Horvath — but Williams's latest Harper's Bazaar cover [1] is a serious departure from her buttoned-up character. For the magazine's December/January 2016 issue, Williams showed off her insanely fit physique in belly-baring crop tops and a cool selection of denim and leather.

The stunning editorial is personally significant for Williams, who expressed being self-conscious about her stomach. On her Instagram [2], Williams shared a photo from the shoot and wrote, "I am particularly proud of this one. For my whole life until 2 years ago, I was super self-conscious about my stomach. I dreaded bikini season. I kept it covered. Boyfriends weren't allowed to touch it. No amount of crunches seemed to strengthen it."

Her turning point? Discovering Pilates. She added, "Then I met Ilaria Cavagna [3] who introduced me to the magic and focus of Pilates. I have never felt stronger, healthier, more focused, or more feminine. Now, I love my stomach because it keeps me grounded to my core. And it's still the same stomach! Here's to proving yourself wrong."

Williams is one of many celebrities, like Kerry Washington [4] and Lea Michele [5], to publicly praise the core-burning workout as of late (try it yourself with our two-minute Pilates ab workout [6]). While we can totally get behind the transformative workout, we are surprised by her activewear advice. In Harper's Bazaar [7], she said: "This is my gift to you: Spanx makes the most incredible exercise spandex. I wear them almost every day. They have a little control top, so they're flattering."

Related: The Ultimate 30-Minute Cardio Pilates Burner! [8]