Heat 'til steaming but not boiling, stirring occasionally with whisk to prevent sticking:
2 cups whole milk
2 oz sugar
Meanwhile, combine in a large bowl:
1 1/8 oz cornstarch
2 oz sugar
a pinch of salt
And, once combined, add in:
5 egg yolks
When milk is hot, stir a small amount into the egg yolk mixture, then stir in the rest. Whisk vigorously, then dump back into the saucepan. Heat, stirring constantly, until mixture bubbles (splurts, really).
Take off the heat and add:
1 1/2 tsp vanilla (ish - you may want to start with 1 tsp and add more to taste)
2 Tblspn butter
Whisk until thoroughly incorporated. If lumps exist (usually when one is lazy in the earlier combining-things-together steps), and it's important that lumps not exist (or if it's for a wedding cake and it's just sufficiently important that lumps not exist *at all*), press through a strainer while still hot.
If you're using it for something where a "skin" on the pastry cream doesn't matter, gloop it into containers, cool slightly, then refrigerate. If you need to avoid a skin, whisk occasionally until mixture is no longer very, very hot. Transfer to the receptacle of your choice and press plastic wrap to the surface of the pudding/pastry cream, then refrigerate. Before using, whisk a bit to smooth it out.
If using this to stabilize whipped cream as a cake filling:
Whip cream. Whisk pastry cream to smooth it out. Fold 1/3 of the whipped cream gently into the pastry cream. Fold the remainder of the whipped cream extra-gently in. Run a whisk through to check that it's all incorporated, and fill the cake (or cream puffs) fairly soon (you don't need to work as fast as with, say, egg white-based things, but it's still wise to use it immediately).
It should be refrigerated, in general, but will hold up much, much longer than straight whipped cream will (it has been observed to remain light and fluffy three days; then the last of the cake was eaten, so I don't know how much longer it would have lasted). I used approximately equal parts cream (before whipping) and pastry cream; proportions can be varied significantly, though.