Sift the flour and custard powder in a large mixing bowl.
Place butter in the bowl of your stand mixer ( you can use your hand mixer or a whisk too), sift in powdered sugar, beat on low heat initially, until the powdered sugar is mixed with the butter, then increase the speed to medium and beat till light and creamy.
Add cardamom powder, rose essence, chopped almonds, and pistachios. Mix well.
Now switch to the lowest speed and add the flour in 3 parts incorporating the butter into the flour.
Gently bring the dough together. Do not knead.
If at all the dough is dry, sprinkle few teaspoons of milk at a time, do not add too much.
The dough should be soft.
Shape the dough into a log and wrap in a cling film. Refrigerate it for 3-4 hours or overnight.
Preheat the oven to 180 C /350 F.
Line a baking sheet with parchment paper.
Remove the log from the refrigerator and cut slices about a quarter inch thick.
Lay the cookies on the sheet at least 2 inches apart as they expand when they bake.
Sprinkle the remaining 1 to 2 tsp of chopped almonds and pistachios, press it in lightly.
Bake for 12-15 minutes.
When the bottom of the cookies begins to brown, they are done.
They will be soft in the middle, but will firm upon cooling.
Cool in the baking tray for five minutes and then transfer to the wire rack to cool completely.
The cookies stay good for two weeks when stored in an airtight container.