Step 1

Whip the egg yolks and the sugar together until the yolks are light and frothy. Whisk in the cornstarch. Heat the milk in a saucepan until just below boiling and take it off of the heat. Pour 1/3 of the milk into the egg yolks and cornstarch, whisking constantly. Then whisk the remaining 2/3 of the milk into the mixture. It should be very liquid at this point. Return the cream-yolk mixture to the saucepan and heat on medium heat. Whisk continuously throughout this process. Continue whisking and heating until the mixture thickens and begins steaming, about 5 minutes. Skip this step if not using gelatin. Bloom the gelatin in 1/4 cup of cold water by quickly whisking the gelatin into the water so no clumps remain. Leave to rest and bloom for 5 minutes. Add the gelatin to the hot pastry cream custard and whisk until fully combined.

Step 2

Allow the pastry cream to fully cool to room temperature before proceeding to the next step. Working with chunks of butter at a time, use an immersion blender to blend the softened butter into the cream. You can reserve the mousseline cream in the refrigerator in an airtight container until ready to use. Before using, allow the cream to come to room temperature and whip for 3 minutes to lighten.