I thought it was flour too, but I just checked my pastry chef books from school, and it is cornstarch. Once finished, lay a piece of saran wrap directly pressing ontop of the pastry cream and put in fridge. You can make it a few days before hand also. 
It's good stuff!!!
Yes you can replace for the same amount.
Here is a cool link with a recipe and a demo.
http://www.frenchpastrychef.com/pastry/1201pastrycream.shtml
MILK 1 qt
YOLKS 7
SUGAR 250g
CORN STARCH 90g
BUTTER 112g
This is a tried and true french recipe! And the link has perfect instructions. It's cool.
I thought it was flour too, but I just checked my pastry chef books from school, and it is cornstarch.
Interesting. Fannie Farmer and Joy of Cooking both use flour. (I've just checked again as well.)
I believe the use of flour may be an american form of making it, and perhaps a faster easier way for the home cook to get a thicker custard without burning it. The process is different with flour, at least in the wilton recipe.
The corn starch is perhaps the european method of making pastry cream. That recipe is French. The Spanish also make a similar custard using even more cornstarch as a thickener. Served like brulee but cooked like anglaise.
Yes, the cornstarch thing is French. The books I was referring to are Professional Baking Le Cordon Bleu. This is what the books call for but you can also substitute flour as well. MILK, SUGAR, EGG YOLKS,WHOLE EGGS,CORNSTARCH,SUGAR,BUTTER, VANILLA BEAN OR EXTRACT. ....So, Fannie Farmer and Joy of Cooking must be doing it the american way...I think "Jesaltuve" explained it perfectly.
I would skip the "brushing with butter" stage and just use plastic wrap pressed to the surface, like mentioned above.
My instructor calls it "truckstop pastry cream" (the version made with cornstarch, that is), LOL. But it's what we use in our recipes in class. Never used flour, though.
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