Start with the pastry cream

In a large enough bowl with the help of a Hand whisk, whisk together sugar and egg yolk until slightly fluffy for about 1-2 minutes

Mix in corn starch, flour, and vanilla until throughly incorporated and have a smooth paste

Bring milk to simmer in a saucepan on medium heat then remove it from the stove.

Pour the warm milk over the egg yolk mixture slowly while whisking vigorously with a Hand whisk.

Pour the mixture back into the saucepan and cook on medium heat for a few minutes until it thickens. Concretely it is approximately 1 min after the first boil. It might get lumpy first, dont worry just keep whisking and the cream will get smooth and glossy

If in doubt, strain the pastry cream for a lump free, creamy end result.

Add in room temperature soft butter and mix with a rubber spatula until completely incorporated. It is worth to add the butter in 3 stages and mix well after each addition.

Place the cream into a shallow bowl and cover the entire surface with plastic wrap to avoid skin forming on top. Let it come to room temperature

Make the mousseline cream

Crème mousseline can be made with room temp pastry cream and room temp butter. Make sure everything is at room temp otherwise the mixture might curdle

Once pastry cream cooled to room temp, prepare the mousseline cream by whipping room temp butter first for 1-2 minutes. Start adding in the room temp pastry cream in 4 stages until cream reaches a stable yet fluffy texture.

Do not over whip the mixture as it can become runny or can curdle. The cream mgiht be on the soft side at this point due to the room temp butter used so place it back into the fridge for an hour to chill before using it.

Store refrigerated for 2-3 days.