Begin by lining an 8x8 inch square baking pan with parchment paper, leaving some overhanging on the sides to facilitate easy removal after the fudge has set.
In a medium-sized saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, unsweetened cocoa powder, instant coffee granules, pure vanilla extract, and salt. Place the saucepan over low heat.
Stir the mixture continuously to ensure even heating and to prevent burning, until the chocolate chips are completely melted and the mixture has transformed into a smooth, glossy consistency. This should take about 5-7 minutes.
If you are using chopped walnuts or pecans, gently fold them into the fudge mixture at this stage, making sure they are evenly distributed throughout the mixture.
Carefully pour the luscious fudge mixture into the prepared baking pan, smoothing it out evenly with a spatula to ensure a flat surface.
Place the pan in the refrigerator and allow the fudge to chill for at least 2 hours, or until it has firmed up completely.
Once set, use the parchment overhang to lift the fudge out of the pan. Cut it into neat squares with a sharp knife for serving.
Notes
For a decorative touch, dust with cocoa powder or wrap individual squares in foil for gifts.