Possibly: Dan Povenmire; Jeff "Swampy" Marsh; Danny Jacob; Keith Ferguson or Tyler Mann
Mortimer with Baby Heinz
"When they are born, he slaps their bottoms to start their breathing." — Mortimer Shlussel[source]
Mortimer Shlussel is a doctor who works in the Maternity Ward of Gimmelshtump Memorial Hospital. He is notable for having been the only person present at the birth of Heinz Doofenshmirtz.
Doctor Shlussel has been working at Gimmelshtump Memorial Hospital for over 47 years, and was the doctor who delivered Heinz Doofenshmirtz and gave him the "spank of life". It is unknown if Mortimer ever interacted with Roger Doofenshmirtz.
In his youth, He had hair of a dark color, most likely black. 47 years later, his hair had turned white. Shlussel has never been seen without his nurse's outfit.
Personality and traits
Doctor Shlussel appears to have difficulty differentiating between babies and adults, as he spanked a full-grown Heinz Doofenshmirtz and declared, "he is breathing", despite Doofenshmirtz not requiring such care.