Sift the flour and cocoa powder in a sufficiently 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 vanilla extract and mix well.
Now switch to the lowest speed and add the flour. Gently bring everything together until the dough is soft. Do not knead. If at all the dough is dry, sprinkle few teaspoons of milk at a time, do not add too much. I didn't require any milk.
Add the chopped cashew nuts and fold in the cookie dough.
Divide the dough into two parts. 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 degrees C/350 degrees F. Line two baking sheets with parchment paper. Or bake in two batches. Remove the log from the refrigerator. Keep it on the counter for 5-10 minutes. Using a sharp knife, cut 1/4 inch thick slices.
Lay the cookies on the sheet at least 2 inches apart as they expand when they bake. If you are baking in two batches, keep the sliced cookies in the refrigerator, let the baking sheet cool completely, then place the second batch of cookies and bake.
Bake for 13-15 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.