Crème mousseline consists of pastry cream and butter, whipped to a light and airy texture. It is used as a filling or covering in a range of classic French pastry and for the famous Hungarian cake called Esterházy Torte. It is a rich and creamy custard-like buttercream. Once chilled, crème mousseline becomes very firm; this makes it an excellent filling for cakes that must be left unrefrigerated or that are served in a warm environment.
Preparation time 30 minutes (plus cooling time) Cook time about 5 minutes Makes about 915 grams (2 pounds or about 4 cups)
475 milliliters (2 cups) milk
100 grams (½ cup) sugar, divided
½ vanilla bean, split lengthwise
5 egg yolks, at room temperature
40 grams (⅓ cup) cornstarch
210 grams (7.4 ounces or 15 tablespoons) butter, cut into pieces and at room temperature
Measure all of the ingredients.
Place the milk and half the sugar in a heavy saucepan. Using a sharp knife, scrape the seeds from the vanilla bean and place the seeds (and the bean itself) into the milk. Place the pan over medium-high heat. Bring to a boil and remove from the heat.
In a medium bowl vigorously whisk the yolks, the remaining sugar and cornstarch together until well blended and light in color. Whisking continuously is critical to avoid having the sugar “burn” the eggs by dehydrating them and making it difficult to incorporate them into the milk in the next step.
While whisking constantly, pour about a third of the boiled milk into the egg mixture – to temper it. Be sure to whisk vigorously or the eggs will curdle and you will need to start again. Then whisk the tempered egg mixture into the remaining milk in the saucepan.
Place the egg-milk mixture on high heat and whisk constantly, taking care to scrape the bottom of the pan to prevent scorching. As the mixture starts to thicken it will look lumpy, but if you whisk it vigorously, it will smooth out. Boil (again, while whisking constantly) for 2 minutes until thick and smooth. The 2 minutes is necessary to fully cook and activate the cornstarch.
Remove the pastry cream from the heat and pour it into a shallow bowl or pan, spreading it out with a spatula to aid the cooling process. Remove the vanilla bean from the cream and discard it. Cover the surface of the cream with plastic wrap - directly on the top - to prevent the formation of skin on the cream.
Set aside to cool until slightly warm.
Place the still-warm pastry cream in a mixing bowl and whisk it vigorously until smooth. Slowly add the butter, beating until completely incorporated and the mixture is light and fluffy. Scrape down the sides of the bowl with a rubber spatula as needed. The key to making this cream light and fluffy is having the butter and the pastry cream at the same temperature. The butter should not be added if the pastry cream is still very hot or it will melt and separate. If the pastry cream is too cold, the butter will not incorporate. If either of these things happen either refrigerate (if too warm) or heat up (if too cold) until the desired temperature is reached and then beat until smooth.
Use the crème mousseline as intended as soon as possible after it is prepared. It is best to use it immediately.
Wrap well and refrigerate any leftovers. Warm slightly and vigorously beat again until light and fluffy before re-using to fill or frost a cake.