Line a 12 cm square baking dish with foil or parchment paper and set aside. Take pistachio on a chopping board and chop it roughly.
Now in a microwave safe bowl, take chocolate chips, butter and condensed milk. Mix once.
Place this in a microwave and set the timing for 1 minute. Remove it after 1 minute and mix well using a spatula, if the chocolate is not melted completely, then pop it back into the microwave for 30 seconds.
Keep stirring. By this time the chocolate would have melted, if not pop again for 30 seconds. Now add in the pistachios and mix once.
Pour this mix into the baking dish and smooth the top. Place this in the fridge for 1 hour or in the freezer for 30 minutes.
Remove it and invert to peel off the foil. Cut into cubes. Serve.