Process the pistachios and milk powder in a food processor until fine powder, slightly moist, is formed. About 3-4 minutes.Use a rubber spatula to scrape the bottom and sides of the bowl 3-4 times.
Drizzle in the oil and process until blended.
Add the sugar and mix to incorporate. The longer you process, the thinner the paste will become.
Transfer the cream to a large pastry bag fitted with the filling tip. If the cream is warm, refrigerate it for 10 minutes.
Assemble
Bring the sugar and water to a boil over medium heat, then reduce to low and simmer for 4-5 minutes longer.
Insert the filling tip into the bottom of the pastry on the side facing you, and gently press the filling in, angling the tip at 45 degrees towards the center.
Repeat with the opposite side.
Place the pastry upside down for a few minutes to allow the filling to fill it. If some filling is oozing, use a knife or an offset spatula to smooth it.
Using a pastry brush, brush the pastries with the simple syrup.
Place the chopped pistachios in a wide bowl, and coat the pastries with the chopped nuts.
Serve.
Notes
By folding 1/2 cup of the pistachio cream into crema pastiera, you can turn it into a decadent pastry cream for an even richer filling.
The cream recipe works well with nuts, including hazelnuts, pecans, and walnuts.
This recipe yields eight large pastries or 12-15 small ones.
Store at room temperature for two days.
Store the cream in the refrigerator for up to 10 days.
To freeze, wrap in plastic wrap, place in a freezable bag and freeze for up to six weeks.