1. Line an 8×8 baking pan with aluminum foil and spray it generously with nonstick spray. Set aside while you combine together the condensed milk and white chocolate chips in a small saucepan. Place the saucepan on the stove over medium heat, and stir regularly until the chocolate is completely melted and the mixture is smooth.
2. Remove the saucepan from heat. Add in the maple extract and stir to combine, then add in the walnuts and the pinch of salt. Give everything another stir, then pour the mixture into the prepared baking pan. Smooth the top over with the back of a spoon, then place the pan into the fridge. 
3. Chill the fudge in the fridge for at least 4 hours, but preferably overnight. Remove the pan from the fridge, then use the overhanging aluminum foil to lift the fudge out of the pan and on to a cutting board. Cut the fudge into 36 equal pieces (or a size you desire). Store the fudge in the fridge or freezer.

Fudge will keep at room temperature for a day if you have it out on a serving tray, but it keeps best when cold. It can be stored in the fridge for up to 1 week or in the freezer for 2+ months.