Diplomat Cream or Crema Diplomatica in Italian is nothing else but a combination of classic pastry cream and Chantilly cream.
Turn the heat on low and slowly start pouring in egg yolk batter.
Be careful not to overcook the cream otherwise you’ll get a sweet omelet. Once you see consistency of the cream is thick enough transfer, add butter, whisk until it's completely incorporated.
Transfer the pastry cream in a large bowl, cover with a plastic wrap making it touch the cream. Let cool completely.
With a mixer and a whisk attachment beat the cream until foamy.
Add powdered/confectioners sugar, whip cream stabilizer (optional) and continue beating on medium speed until stiff peaks. It should take about 5-7 minutes.
Be careful not to overbeat the cream. Once you feel the cream is ready do a spoon test, if the cream stays on the spoon turned upside down it’s ready.
It’s best to chill the bowl and the whisk attachment in the fridge before beating the cream.
Keep in mind that if the cream is not beaten enough it might start to “melt” if left in a warm place.
That’s why I strongly suggest you use whip cream stabilizer.
Whisk gently but thoroughly. Add the remaining Chantilly cream.
Transfer in the pastry bag and use for cakes, muffins, tarts and more.