Miranda is preparing to deliver her songs with a slice of pie.  Colleen Ballinger — aka YouTube sensation “Miranda Sings” — will make her Broadway debut in “Waitress.”  She’s bouncing back from the cancellation of her Netflix series “Haters Back Off,” by taking on the role of Dawn for a four-week limited run. The actress-comedian steps into the Tony-nominated musical from Aug. 20 to Sept. 15 at the Brooks Atkinson Theatre.  Ballinger, 32, is a classically trained singer who racked up more than 100 million viral video views as her cluelessly untalented alter ego, Miranda Sings.  The triple-threat geeked out Friday while tweeting the news of her Great White Way gig: “I’M GOING TO BE ON BROADWAY!!!”  “You don’t understand how exciting this is for me …,” the new mom says in an emotional video posted to her official YouTube channel. “This has literally been my dream since I could speak … I never thought this would happen to me. I thought it was so out of my reach.”  Singer-songwriter Sara Bareilles’ musical opened in 2016 and is inspired by the 2007 indie movie of the same name starring Keri Russell and written by the late Adrienne Shelly, who was murdered in 2006 before the acclaimed film’s release.  The long-running stage adaptation tells the story of Jenna, a server and pro pie-maker who dreams of a way out of her small town and rocky marriage. Ballinger will play Dawn, a fellow waitress who’s one of Jenna’s only pals.  The current Broadway cast of “Waitress” — which also has a national company touring the US — features Shoshana Bean (“Wicked”) as Jenna, Erich Bergen as Dr. Pomatter, Charity Angel Dawson as Becky, Ben Thompson as Earl, Larry Marshall as Joe, Benny Elledge as Cal and Noah Galvin as Ogie.