Diplomat cream is what you use when you want the flavor of pastry cream but in a lighter, fluffier package. It makes an outstanding filling for all sorts of things that aren’t baked afterward (or are only finished in the oven or broiler, like brioche polonaise). The whipped cream it contains can’t take heat.
I know what you’re thinking: just how much whipped cream is in this, Joe? Well that’s up to you. Diplomat cream can be as light as 1-1 pastry cream to whipped cream. My ideal is 2-1 pastry cream to whipped cream. But you can go heavier or lighter as you see fit. I generally go with a less-sweet pastry cream and a slightly sweetened whipped cream, so the overall effect isn’t too sweet. If both components are just barely sweet enough, the finished product will be about perfect. Put the two in a bowl:
Fold gently together.
And bingo, you’re done! Use it right away.