In a large bowl, stir together the powdered gelatin and 3 tablespoons of milk. Set aside to bloom for five minutes.
Place the egg yolks in a medium bowl and set aside.
In a medium sauce pot (preferably with a heavy thick bottom) pour in the rest of the milk. Bring the milk just to a simmer - stirring frequently to prevent it from burning.
While it is heating up, add the sugar into the egg yolks and whisk until the mixture starts to lighten and become frothy.
Once the milk is hot, remove it from the heat.
Whisk the egg yolks constantly and use a ladle to slowly stream in 1/2 a cup of the hot milk. This will temper the eggs.
Then switch and start whisking the remaining milk in the pot and slowly pour in the tempered eggs.
Return the pot to medium heat and use a rubber spatula to stir the pot constantly making sure to scrape the edges of the pot.
Cook the custard for about 3-5 minutes until it begins to thicken and coats the back of a wooden spoon. Another thing to look for is that most of the bubbles on the surface dissipate.
Once cooked, pour the hot custard through a sieve into the bowl with the bloomed gelatin. Then, add in the vanilla extract and whisk the gelatin into the custard until it has fully dissolved.
Set the bowl aside and stir regularly to help it cool evenly.
Once the custard is cooled and thickened, whip the heavy cream to stiff peaks.
Add in half of the whipped cream to the custard and use a rubber spatula to fold the two together.
Pour the whipped cream and custard mixture into the remaining whipped cream and fold again to combine being careful not to knock all of the air out.
Pour the mixed Bavarian Cream onto the vanilla cake and use an offset or rubber spatula to spread it evenly across the cake, all the way to the edges of the pan.