Prepare an ice bath in a large bowl--it should be big enough to hold the saucepan.
Place the saucepan over medium heat and add the milk. Bring it to a simmer, then remove from the heat.
In a bowl, beat the sugar and eggs until the sugar starts dissolving, then whik in the cornstach.
Slowly pour a half cup of the warm milk into the bowl with the sugar and eggs, then gradually pour this back into the saucepan, whisking constantly--you're tempering the eggs here, so they won't ccurdle.
Return the pan to medium heat and whisk until the custard thickens a bit.
Set the pan into the icebath and add the butter. Stir or whisk until the butter melts and the pastry cream is smooth. Then stir in the vanilla.
Cover with a sheet of plastic wrap and chill until cool, then it's ready to use.