What is creme mousseline?

Creme mousseline is a very uncomplicated story. It is a simple pastry cream mixed with softened butter. There are various ways to make it: one recipe states that you should put butter and cream in a bowl and mix it for 10 minutes. Another likes to start by whisking the butter and then adding cream. Still others start with cream and then adding butter. No matter how you do it, there is one golden rule - THE INGREDIENTS MUST BE at room temperature. So, when making the creme, I like to take out butter just as I start making the cream. Give the butter a few hours to soften; during that time, I prepare the cream and let it cool. 

Only when both are ideally the same temperature can you mix them together. It can curdle if one is colder or cool, and we do not want that. Haste is the biggest enemy here, so take your time and be prepared to enjoy a delicious cream you can use in tarts, eclairs, mille feuille, cakes, and other desserts. 

500 ml milk
pinch of sea salt
1/2 vanilla pod - pod and seeds
4 large egg yolks
1 medium egg
150 g light brown sugar
35 g cornstarch
30 g butter + 220 g

Pour milk into a pot and add salt and vanilla. Heat until the milk is hot but not bubbling.

In the meantime, mix the eggs, sugar, and cornstarch in a bowl with a whisk to incorporate. When the milk is hot, continue to stir the eggs and add a bit of heated milk. Stir. Then add a little more and stir. Do so 2-3 more times. When the egg mixture is more liquid, pour it into the pot with the remaining milk.

Put the pot back on medium-high heat. Stirring continuously, heat until the cream thickens. When you get big, slow bubbles, keep stirring for another 20 seconds. Take off the heat, add 30 g of butter, and mix in until incorporated. Then pour the cream into a clean bowl and cover it with cling film, placing it DIRECTLY on the cream, not on the bowl. Allow to cool at room temperature, but do not refrigerate. If you don't have enough time, put it in a container with ice and cool it faster by stirring. Set aside 4-5 generous tablespoons. 

Add the rest of the butter to a baking bowl. Cream for a minute or two to make it fluffy. If the butter is very warm, it takes less time. If the room temperature is colder, it might take around 3-4 minutes. Add 1/3 of the cream with the mixer running to incorporate. Add the next third, combine, and then the last third. Incorporate, turn the speed to medium-high, and mix for approx 7 minutes. In the end, you can put the mixer to the almost highest speed and mix for a minute more. Spread the cream in a shallow dish, cover with plastic wrap, and leave to rest for 2 h in the fridge to the consistency ready to pipe.