Britney Spears put a cap on her birthday week with a little booze, a lot of cake and a middle finger for her haters.

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.