Microwave chocolate in a bowl for 20 seconds. Stir and continue to heat in the microwave in 10 second intervals till most, but not all of the chocolate is melted. Stir the chocolate until it is smooth, shiny and completely melted. (Took me about 30 seconds).
Add condensed milk to the melted chocolate mixture and stir well. Microwave for 2 minutes and stir vigorously. Microwave for another 2 minutes and then add chopped nuts and mix well.
Spread the mixture in an 8-inch square pan lined with parchment paper. Add the colored chocolate chips (if using) evenly on top of the mixture and gently press them in using the back of the spoon.
Refrigerate for an hour and then cut into squares.