Spray a 2-quart slow cooker with a baking spray like Bak-Klene. Set the slow cooker to a low setting. Add the sweetened condensed milk, chocolate chips, butter, and vanilla extract to the slow cooker. Give it a stir to combine. Cook with the lid OFF, on low for about 1.5 hours, stirring every 30 minutes.
Line a small casserole dish or 7-inch pie plate with parchment paper and set aside. Crisscross parchment paper strips to make it easy to remove and lift out of the dish.
When the fudge is well combined, check to see if it's easy to stir with a wooden spoon then stir in the chopped Walnuts. Pour the fudge into the prepared dish and smooth out the top of the fudge.
Refrigerate the fudge until set, about 2 hours. Remove from pan and cut into bite-size pieces.
Notes
Leave the lid off the slow cooker to avoid condensation from getting into the fudge mixture. Water will add an unpleasant grainy texture.
Dark chocolate or a mixture can be substituted.
If doubling the recipe use one (1) full can (14-ounce) of condensed milk.
The nuts are optional or can be substituted with pecans or another nut of choice.
Use a hot knife to easily cut the fudge into clean-edged squares.