The former Made In Chelsea star donned the gorgeous number - a silver studded body with a sheer skirt - on Friday (9 May) and it left very little to the imagination!
Millie, 24, told fans: "I just died and went to dress heaven @julienmacdonald Trying this on made me want to be Beyoncé."
She then added: "Such a fun afternoon @julienmacdonald with @officialmason giving fierce advice #beyoncefortheday #ifonly."
Does this mean Millie wants to hit the stage and sing her heart out a la Queen Bey? Watch this space!
However, Millie rounded off her day of trying on seriously sexy dresses in a less than diva-like way.
After paying a visit to the home she and hubby Professor Green are currently doing up, and getting excited about her new oven, Mills opted for an early night.
Millie said: "So happy to be having a night in #nofomo"
Then she added: "So excited our gaggenau ovens have arrived! #countingdownthedays."
We can't imagine Beyoncé cooing over a new oven, tbh.
Still, Mills has been crazy busy jetting around on holiday and partying like there's no tomorrow, so we reckon she deserved a Friday night in!
And while the rest of us may have spent our Saturday morning hanging *guilty as charged* Mills was reaping the benefits of an early night.
She re-Tweeted a yoga pal on Saturday, who said: "Feeling cleansed after this morning's Hot Yoga with @yogahaven Nice to see @millsmackintosh practising with us too."
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