Add condensed milk and chocolate chips to a microwave-safe bowl.
Put the bowl in the microwave and heat it for a minute.
Stir the chocolate until it is smooth, shiny, and completely melted. If the chocolate chips are not completely melted, microwave for 10 more seconds and stir it again.
Add vanilla extract and cocoa powder (if using) to the mixture. Mix well.
Add about 1/4 cup of chopped nuts and mix well. Reserve the other 1/4th cup for garnishing.
Spread the mixture in a loaf pan lined with parchment paper. Add the remaining chopped nuts 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.
I prefer using semi-sweet chocolate chips in the recipe and it brings amount the right amount of sweetness in this recipe when combined with condensed milk.
You can also use dark chocolate chips or do a mix of dark and milk chocolate chips. I would avoid using only milk chocolate morsels because that would make the recipe really sweet.