The chocolate cake, studded with candles in the shape of miniature bottles of champagne, read 'Happy Birthday Bruno' and has since been re-tweeted 163 times and 'favourited' by 657 of Nigella's fans.
Many of her followers replied to the tweet with positive messages about her return.
"Good to see you back," wrote one user. "Hope Bruno had a wonderful birthday."
Nigella returned to Twitter with a photo of her son's birthday cake
Nigella's fellow celebrity chef, Gizzi Erskine, was also pleased to see the Nigellissima author back online.
"second this xxxxxxx" wrote Gizzi, responding to a Twitter user who said "Welcome home Lady."
The picture of Bruno's cake is in keeping with Nigella's usual tweets, which include images of her culinary masterpieces of a near daily basis.
Bruno is the younger of Nigella's two children. She also has a daughter, Cosima, both from her marriage to former journalist, John Diamond, who died from throat cancer in 2001.
Nigella's relationship with her second husband, Charles Saatchi, became the subject of attention two weeks ago when shocking photograph were published of art collector Charles gripping his wife's throat during an argument.
Charles, who is worth a reported £100million, said the photos depicted a "playful tiff" and that Nigella was unharmed.
Nigella Lawson and her husband Charles Saatchi
"About a week ago, we were sitting outside a restaurant having an intense debate about the children," claimed Charles. "And I held Nigella's neck repeatedly while attempting to emphasise my point," he told the Evening Standard.
"There was no grip, it was a playful tiff. The pictures are horrific but give a far more drastic and violent impression of what took place. Nigella's tears were because we both hate arguing, not because she had been hurt."
Charles has since been cautioned for his actions and late last week Nigella was seen moving her things out of the family home.
The TV chef had removal men take belongings from the £12million house in Mayfair that she had shared with her millionaire husband and has also been spotted without her wedding ring on.
Nigella has been seen without her wedding ring on
Prior to Nigella's tweet on Saturday, the only word from the TV chef was in a statement last week from her publicist which confirmed she had "left the family home with her children".
"Her heart is bleeding right now," a source close to Nigella told The Sunday People. "She wants out.
"Neither she nor Charles are interested in counselling or trying to save the marriage. It seems they’re done and she is broken and desolate."