In a medium saucepan, combine milk, half of the sugar, vanilla bean, and salt. Cook over medium heat until mixture comes to a simmer.
In a medium bowl, whisk together egg yolks, remaining sugar, and vanilla bean seeds. Whisk until sugar has dissolved slightly.
Whisk in the cornstarch until well combined.
Whisking constantly, slowly pour about 1/2 cup of the hot-milk mixture into the egg-yolk mixture, 1/2 cup at a time, until it has been incorporated.
Pour mixture back into saucepan, and cook over medium-high heat, whisking constantly, until it thickens and registers 160 degrees on an instant-read thermometer, about 2 minutes.
Remove and discard vanilla bean. Transfer to a mixing bowl. Add the butter, and whisk well until the butter melts and the mixture cools, about 5 minutes. Cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent a skin from forming. Refrigerate until chilled.
Fold 3/4 cup of the pastry cream into the unsweetened whipped cream until thoroughly mixed. Fold in the sprinkles.
Glaze
In a small mixing bowl with a whisk, mix the powdered sugar, salt, and hot water thoroughly.
Assembly
Place the pastry cream into a disposable piping fitted with a small round or small star tip. Flip the croissant onto the back fill it with the pastry cream in three spots (the two ends and middle).
Dip the top of the croissant into the glaze, place on a piece of parchment paper, sprinkle with rainbow nonpareils and allow to set, about 3 minutes.