In a medium bowl, whisk together egg, egg yolk, sugar, cornstarch, and salt.

In a small saucepan, heat milk, vanilla bean paste, and cherry juice or liqueur over medium heat, stirring frequently, just until steaming. (Do not boil.) Slowly add half of hot milk mixture to egg mixture, whisking constantly. Whisk egg mixture into remaining milk mixture in saucepan; bring to a boil over medium heat, whisking constantly. Cook, whisking constantly, until mixture is thickened, 2 to 3 minutes.

Strain custard through a fine-mesh sieve into a medium bowl, discarding solids. Whisk in butter until melted and smooth. Cover custard with plastic wrap, pressing wrap directly onto surface of custard to prevent a skin from forming. Refrigerate until thick and cold, at least 2 hours.

In the bowl of a stand mixer fitted with the whisk attachment, beat cold cream at medium-high speed until stiff peaks form, 2 to 3 minutes.

Whisk cold custard to loosen; stir one-fourth of whipped cream into custard. Gently fold remaining whipped cream into custard until combined. Use immediately.