Check out sweet photos from their epic celebration
Credit: Source: Gwen Stefani/Snapchat

Blake Shelton kicked off his 40th birthday Saturday, with a sweet message from girlfriend Gwen Stefani.

“Happy birthday to my favorite person I ever met @blakeshelton,” the 46-year-old singer wrote on Twitter.

She also punctuated the post with the emojis of a cow, a bee and a sunflower, calling Shelton a “superbabe” in the tweet’s hashtag.

Stefani even went as far to change her Twitter avatar to a picture of her 40-year-old beau, adding to the celebration.

“Now THIS is how you start your 40’s!!!!!!!!,” Shelton wrote, retweeting the photo. “Thank you @gwenstefani for everything…”

The loving message comes two days after Stefani was seen picking up party supplies at West Hollywood, California, novelty shop, Ahhs.

The duo started dating seven months ago, and are said to be “really happy together,” an insider told PEOPLE.

They “not putting pressure on themselves” and are “just enjoying life together,” the insider said, adding that their friends see a “longterm future” for the couple, who made their relationship red-carpet official over Oscar weekend.

“Everyone is thrilled because they’re both incredibly nice people,” another source told PEOPLE.

The “Hollaback Girl” singer continued her birthday wishes for her fellow Voice coach, positing another childhood photo of Shelton on her Instagram account.

“Happy birthday to @blakeshelton literally my favorite person,” she captioned the shot.

Stefani signed both of her social media messages with her signature “gx” – known to mean “Gwen, with a kiss.”

Stefani celebrated with Shelton, her brother Todd and sister-in-law Jill and documented everything from the guitar-shaped cake and personalized wine glasses of the singers to Shelton’s birthday accessories – “Happy Birthday” glasses and a “40” crown – on her Snapchat. (The group got the celebration started on a private jet to Colorado and then later in Shelton’s dressing room.)

The party continued when Shelton took the stage Country Jam in Grand Junction, Colorado on Saturday night. After sharing a few videos from backstage, Stefani unexpectedly joined the birthday boy on stage for their song “Go Ahead and Break My Heart” at the end of his set.

The No Doubt singer then played her band s song “Hella Good.” She left the stage to let Shelton complete his encore, but not before they shared a quick smooch.

Earlier this month, Stefani set the record straight about all the “gossip-y, weird stories people make up” about her relationship with Shelton, telling 94.7 Fresh FM’s “The Tommy Show” that she’s taking their relationship “one day at a time.”

Both musicians ended long-term relationships last year, with Shelton and Miranda Lambert divorcing in July and Stefani and husband Gavin Rossdale announcing their split several weeks later.

Related Video: Inside Gwen Stefani and Blake Shelton’s Hot New Romance

Shelton’s birthday wasn’t just filled with romantic messages. Former Voice contestants Raelynn, Cassadee Pope and Danielle Bradbery, whom he’s guided to country success, sent well-wishes too.

Shelton also took time to congratulate friend Luke Bryan on his chart-topping song, “Huntin’, Fishin’ & Lovin’ Every Day,” tweeting him “Congrats @LukeBryanOnline on your #1 song… Even though you are a complete douche bag and a horrible influence on children.. And dumb.”

“And ugly,” he joked in another tweet.

“Just remember – I’ll always be one month younger than your old ass,” Bryan joked back.