• In a medium saucepan, heat the milk and vanilla on medium heat until it is just at a simmer. Don't let it boil.
• While the milk is warming, add the egg yolks, sugar, salt, and cornstarch to a separate bowl. Whisk them together until glossy and smooth. Place the bowl on a dampened kitchen towel to stop it from slipping. 100 g granulated sugar 4 large egg yolks, 100 g granulated sugar, 30 g cornstarch, 1/4 teaspoon salt
• Once the milk is hot, pour it in a steady stream into the egg mixture while whisking vigorously.
• Return the combined mixture to the saucepan over medium heat. Cook the custard, whisking constantly, until it starts boiling. Once it starts bubbling, keep it at a boil and whisk for a full minute.
• Take it off the heat and whisk in the butter until completely combined. 45 g unsalted butter
• Pour the pastry cream into a clean bowl (push it through a fine-mesh sieve first for extra smoothness). Lay plastic wrap directly onto the surface of the custard and let it cool to room temperature. The custard must be at room temperature before continuing, or the butter in the next step will melt. You can refrigerate the custard, but bring it back to room temperature before using.

The butter

• When the pastry cream is cool, beat the softened butter in a stand mixer with the whisk attachment at high speed until it's smooth, creamy, and a lighter color. 170 g unsalted butter
• Add the cooled pastry cream to the butter a bit at a time, whipping at medium speed until you achieve a smooth, airy mousseline cream. Once it reaches that consistency, stop mixing.
• If it's too soft for piping, refrigerate for 20-40 minutes to firm up slightly. Don't chill it too long or the butter will harden too much. If it does become too firm, gently warm the edges of the bowl over a bain-marie and whip it again.