I'll post the recipe I've used in wedding cakes. For any of my wedding cakes, they are in the refrigerator until they go in the car for delivery, and they are delivered no more than about 60-90 minutes before the ceremony or reception. How long they sit at the reception depends on the bride, I suppose, but I've never had any problems this way. Just remind them that they are cream-filled and need to not sit out for 6 hours outside in summer heat, and to refrigerate any leftovers promptly.
This makes 5 cups. I have a note that showed 1/2 batch was enough to fill an 8" round, three-layer cake.
Vanilla Bavarian Cream:
1/3 cup sugar
pinch of salt
1 TB gelatin (this is more than 1 envelope)
5 large egg yolks
1-2/3 cups milk
1 vanilla bean, split (or 1 ts vanilla extract--if using extract, add it after the sauce is cool)
1 cup heavy cream
Refrigerate the mixing bowl for whipping the cream. Have ready a fine strainer near the range, suspended over a small mixing bowl.
In a small saucepan, stir together the sugar, salt, gelatin, and yolks until well blended with a wooden spoon.
In another small saucepan or heatproof glass measure (if using a microwave), heat the milk and vanilla bean to boiling point. Stir a few TB into the yok mixture, then gradually add the remaining milk and vanilla bean, stirring constantly. Heat this mixture, stirring constantnly, to just before the boiling point (170-180 degrees F). Steam will begin to appear and the mixture will be slightly thicker than heavy cream. It will leave a well define track when a finger is run across the back of a spoon. Immediately remove from the heat and pour into the strainer, scraping up the thickened cream that settles on the bottom of the pan. Remove the vanilla bean and scrape the seeds out into the sauce, stir to combine.
In the chilled bowl whip the cream until it mounds softly when dropped from a spoon; refrigerate this.
Cool the sauce in an ice water bath, stirring with a wire whisk until whisk marks barely begin to appear. The mixture will start to set around the edges but will still be very liquid. Whisk in the liquid vanilla at this point, if using. Fold in the whipped cream until just incorporated. The mixture will be soupy like melted ice cream. Remove at once from the water bath and pour into a bowl and refrigerate for at least 4 hours.
Note: It will be very firm when chilled, so I always just sort of *slice* parts of it out of the bowl and place them on the cake layers inside an icing dam, when using it for filling--I've never piped it; it's too firm.