STEP 1: CREAM EGGS & SUGAR
Whisk the eggs and sugar together in the bowl of the stand mixer. Whip the eggs until they are light and pale color and fluffy. You can do this while the milk is heating.

STEP 2: HEAT THE MILK
Cook the milk in a saucepan on hi heat. Bring it to a low boil just starting to bubble (this is just below a boil and boiling point of milk is 203F). Remove from heat. 

STEP 3: TEMPER THE EGGS
With the milk at 185°F (84°C) temper the eggs by slowly drizzling the hot milk into the eggs in the stand mixer while whisking on medium.

STEP 4: COOK THE CUSTARD
Pour the mixture from the stand mixer bowl back into the saucepan.
Heat the saucepan on medium heat and gently bring to a low boil again (just below (203°F (95°C) and cook for 1-2 minutes simmering while constantly whisking by hand. When the custard in the saucepan is thickened, remove the saucepan from heat. 

STEP 5: Pour the custard from the saucepan through a sieve into a clean mixing bowl. Whisk in the vanilla and any other liquors for flavor.

STEP 1: WHISK IN THE BUTTER
Melt in 50g  of diced butter, whisking in a little at a time. Put only a few bits of butter in at a time and whisk gently until it is fully melted in before adding more butter.

STEP 2: COOL DOWN
Cover the mixture with buttered parchment paper touching the top of the mixture. Cover the mixing bowl with a lid or plastic wrap and set on countertop to cool to room temp. (about 30 -45 minutes)

STEP 3: CREAM THE BUTTER
Cream the remaining butter in the stand mixer with a whisk on high speed (level 10) for about 5 minutes until light in color and fluffy. Scrape down the bowl frequently.

STEP 4: COMBINE
The patissiere and the butter should both be at the same room temp (68-72F). Make sure they are the same temp them combine in the stand mixer.

Whisk the creme patissiere  into the creamed butter in the mixing bowl little by little on medium low (L4-5). Make sure each portion is fully incorporated before adding more.

Whip on medium-high (L6-8) for about 5 minutes until creamy but be careful not to overwhip and curdle the pastry cream.

REGULAR MOUSSSELINE OR MERINGUE MOUSSELINE

You can stop at the last step for a regular mousseline or you can mix the mousseline with Italian Meringue. The meringue adds body and stability to the mousseline.

I especially like meringue added for heft if you are going to use it for inner layering in a cake. The mousseline itself will not hold us layer as well.

If you are using this to layer a mille feuille or to fill a puff pastry the meringue is not necessary.

If you need the firmer meringue mousseline, continue to the next step.

TO MAKE THE MERINGUE MOUSSELINE
STEP 1: Make an Italian Meringue
STEP 2: Combine: Fold the meringue, little by little into the mousseline until you have the consistency you want. You don't have to use all of the meringue if you don't need it.