STEP 1: CREAM THE YOLKS, SUGAR AND STARCH
In a mixing bowl or stand mixer bowl, whisk the egg yolks, granulated sugar, salt, and cornstarch until pale, thick, and completely smooth. Whisking the cornstarch directly into the yolks coats the starch granules, preventing clumps and shielding the proteins.

STEP 2: HEAT THE MILK
In a heavy saucepan, bring the milk to a gentle simmer over medium heat until small bubbles form around the rim (~185°F / 85°C). Remove from heat.

STEP 3: TEMPER THE EGGS
Slowly drizzle about one-third of the hot milk into the egg mixture while whisking constantly on medium-low speed to equalize temperatures without curdling.

STEP 4: COOK AND THICKEN
Pour the tempered mixture back into the saucepan. Cook over medium heat, whisking constantly and vigorously across the bottom and corners of the pan. Once it begins to bubble, boil gently for 1 to 2 minutes while whisking to fully gelatinize the cornstarch and deactivate the egg enzymes.

STEP 5: STRAIN AND FLAVOR
Remove from heat and immediately press the custard through a fine-mesh sieve into a clean bowl to catch any cooked egg particles. Whisk in the vanilla and optional liqueur.

STEP 1: WHISK IN THE INITIAL BUTTER
Immediately add the first 50 g of diced butter to the hot pastry cream, whisking gently until completely melted and incorporated

STEP 2: COOL DOWN
Press plastic wrap directly onto the surface of the custard to prevent a skin from forming. Allow it to cool to room temperature (68–72°F / 20–22°C), about 30–45 minutes. (Custard and butter must be the exact same temperature before combining)

STEP 3: CREAM THE REMAINING BUTTER
In the stand mixer fitted with the whisk attachment, beat the remaining 150 g of softened butter on high speed (level 8–10) for 4 to 5 minutes until pale, light, and airy. Scrape down the bowl frequently.

STEP 4: INCORPORATE & EMULSIFY
Reduce mixer speed to medium-low. Add the cooled pastry cream to the creamed butter one spoonful at a time, allowing each addition to fully blend before adding the next. Once incorporated, whip on medium-high for 1 to 2 minutes until glossy, silky, and uniform. Do not overbeat.

STEP 1: FOLD IN THE MERINGUE
If a lighter, airy mousse consistency is desired for cake layering, gently fold prepared Italian meringue into the finished mousseline in two additions until smooth. For mille-feuille or choux filling, omit the meringue for maximum density and clean slicing.