Line a regular muffin tin with paper cups, only 6 are needed. Set aside.
Combine the chocolate and condensed milk in a large, microwave-safe bowl. Microwave in 20 seconds increments and stir after each one until the chocolate has melted. Make sure you stir well, after each interval as the residual heat will melt the chocolate without extra microwaving. This way you avoid overheating and separation of chocolate.
Stir well until smooth, stir in pumpkin puree, walnuts, pumpkin spice, and a few drops of orange gel food color (if using).
Pour mixture into the lined muffin cups and refrigerate for 1-2 hours.
When the fudge has cooled off and harden it can be served.
Make sure you don't add more condensed milk or pumpkin puree, the mixture will become runny and it won't settle. Add more pumpkin spice, for a more prominent flavor. Taste and adjust.