Pastry Cream

Perhaps you could post your recipe?

For me, your ratios are a little bit off from what I'm used to and I mix the flour into the yolk/sugar mixture rather than the milk/cream. Here's how I do it:

I use a little higher ratio of yolks to milk/cream, right at four large yolks per cup of milk/cream, about 1/4 cup of sugar per cup of milk/cream, and 1/4 cup of flour per cup of milk/cream

I heat the milk/cream to 180°F or so, generally with a vanilla bean, and meanwhile, whisk the egg yolks with the sugar until pale yellow and thickened, then whisk the flour into the yolk/sugar mixture. Also, 1-2 tablespoons of soft butter per cup of milk/cream

While whisking, temper with about 1/2 of the milk/cream drizzled into the yolk/sugar/flour mixture, then whisk that mixture into the remaining milk/cream in the saucepan and bring to a boil when it will thicken, whisking continuously to avoid scorching. When thick, remove from heat and, after cooling slightly, whisk in the butter and vanilla if you didn't use the vanilla bean.

I've never heard of this milk/flour/sugar mixture. I've always seen it egg/sugar on the one hand, everything else on the other. Exactly as Pete says, in fact. So I wonder whether it's the ratios at all: the technique looks very weird to me.

I'd guess the problem is the technique.  Some techniques are just asking for trouble, like adding hot liquid to eggs, or adding flour or polenta or any other form of flour to hot liquid. 

Try beating egg with the flour and sugar, heat the milk separately, beat the milk into the egg mixture and heat slowly, stirring, or to be even more certain, beat the cold milk into the egg mixture. The sugar kind of stretches out the egg (i don't have the science but this is my impression) so it doesn't immediately cook into scrambled eggs or egg-lemon soup.  However, i find that if i put cold  milk in, i'm less likely to stand there stirring the darn thing till it cooks since i just don't have the patience and if i'm making a desert it's likely i have a whole dinner going on the fire - and leaving it is inviting disaster.  So i heat the milk, saving me time later.  I beat it into the egg/sugar/flour mixture slowly. Cook it stirring, and stop when it's thick enough - easy to let it cook too much.  The flour should prevent any splitting. 

I stumbled upon http://www.cheftalk.com while looking for some hints regarding making fruit tartlets.  I'm still struggling with the shells, but the pastry cream came out amazing.  Thank you Jill for raising the question and thank you Chef Pete for delivering the answer.