This is Hiromi Iwasaki / 岩崎 宏美 , who emerged on the entertainment scene in Japan in 1975 with a song that propelled her to stardom, ロマンス / Romance .
But according to Wikipedia, her 1978 song シンデレラ・ハネムーン / Cinderella Honeymoon is the song that Korokke uses in his imitations of Hiromi san. From Wikipedia:
Her representative songs are "ロマンス" "Romance" (1975), "思秋期" "Shisyūki" (1977) and "聖母たちのララバイ" "Madonna tachi no lullaby" (1982). However, her most immediately recognizable hit song, "シンデレラ・ハネムーン" "Cinderella Honeymoon" (1978) has been such a consistent feature of monomane talent Korokke's routines, Iwasaki Hiromi has commented that when the intro played in concerts, fans would laugh, so she has ceremonially "gifted" that song to Korokke, who's performance has elongated both their careers.
This clip is Hiromi san performing Cinderella Honeymoon this year: