Decked out in a red jumpsuit and set to Eartha Kitt's "Santa Baby" ... Brit broke out some of her usual dance moves, and seemed to indulge in champagne.

Near the end of the video she puts her whole face in a creamy birthday cake, and happily smears it everywhere.

The celebratory post is one of her first since last weekend's head-turning shout-out to her sister. As we reported, the seemingly sweet note to Jamie Lynn Spears caught everyone off guard, since Britney's been on the warpath against her -- along with their mom and dad -- for years.

BS shut down her IG a few days after that -- leading some to believe the Jamie Lynn love-in was either a sarcastic jab or not really posted by Britney.

She acknowledged her IG pause in Friday's caption, saying ... "Yep it's my b-day all week and I know my Instagram has been down a few times but honestly I did the last two times myself !!! Oops ... I accidentally pressed wrong button."

Not sure her suspicious, and protective, fans will buy that, but she's back ... with a face full o' cake.

BTW, Britney also dropped some early New Year's resolutions -- including her desire to crash a wedding and sing a few songs.