Mix all the ingredients for the custard together off of the heat before putting your pot on the stove. And once you do, make sure to keep stirring on medium-low heat until it thickens up. You don't want to burn or scald the mixture.
Bake the pastries as recommended on the box
Once they have cooled down, pipe each one with the custard filling
Garnish with pistachios and your choice of edible flowers
Drizzle 1/2 tsp of simple syrup on each pastry
Keep refrigerated until ready to serve!