Add the butter to a medium sized, heatproof mixing a bowl and set aside.
Heat the cream and milk over medium heat until a few bubble forms around the edge, then remove from heat and mix in the vanilla.
In a separate bowl, whisk the egg yolks, sugar and cornstarch until glossy and smooth.
Gradually add the hot milk mixture into egg mixture while whisking constantly and then return it all to the pot on the stove over medium heat. Whisk constantly (switching to a rubber scraper now and then, to get into the edges of the pot) until thickened and glossy, around 2-3 minutes.
Pour this immediately through a sieve or fine strainer over the bowl with the butter, pushing the custard through with your whisk or rubber.
Mix the pastry cream with the butter until it's totally melted and combined.
Place a piece of plastic wrap directly over the surface of the custard, cool to room temperature and then chill completely in the refrigerator until ready to use.The pastry cream will keep up to 4 days refrigerated.
Notes
This recipe makes about 2 cups of Creme Patisserie which is enough to fill one 8 inch cake, 8 eclairs or cream puffs and about 6 donuts.
Use a heavy bottom pot to cook your Creme Patissiere to ensure even cooking.
Use a stainless steel pot instead of aluminum as aluminum pots can release color as you whisk, turning your custard gray.
Don’t allow the milk to come to a boil. You want it to just come to a simmer to avoid cooking your egg yolks.
Don’t add the sugar to the egg yolks until just before you add the hot milk, sugar and egg yolks can cause a skin to form if left to sit too long.
If your pastry cream is too thickfor your preference, whisk in a splash of milk at a time and whisk vigorously until it reaches the desired consistency.
Don’t skip straining your mixture. Straining is an important step, as it ensures a silky smooth consistency.
Use your whisk or a rubber spatula to help push the mixture through the sieve and remove any custard that may stick to the bottom of the strainer.
If your Crème pâtissière looks grainy, it was cooked at too high of a heat, causing the egg yolks to cook and curdle. If this happens, even passing it through a sieve won’t help as some of the curdled egg will disperse through the custard making it grainy. Unfortunately, there is no fix for this and you’ll have to try again.
When coolingyour Crème pâtissière, be sure to sit the plastic wrap directly onto the surface of the mixture to avoid a skin forming on top.
Once cooled, it’s always a good idea to stir or whisk your cream to loosen and smooth out the mixture.
Never allow the Creme Patissiere to stay out of the fridge for more than an hour. It can grow bacteria and spoil.