Use the butter to grease an 8-inch square pan. Line your pan with parchment paper. Make sure the paper hangs off the sides of the dish; it'll be easier for you to remove the fudge from the pan if it does. Set the pan aside.
Fill a pot with an inch of water. You want enough water to provide heat, but not so much that the heatproof bowl, when placed in the mouth of the pot, will touch the water. Heat the pot over low heat until the water begins to simmer, and then turn off the stove and place the heatproof bowl in the mouth of the pot.
Put the condensed milk, chocolate chips, and vanilla into the heatproof bowl and stir them with a rubber spatula until they melt. You can also toss in the nuts if you’re using them.
As soon as everything is mixed together, pour the contents into the lined pan.
Smooth everything in the pan and cover with a piece of parchment paper. Chill in the refrigerator for 1-2 hours or until the mixture is firm.
Once the allotted time has passed and the mixture is firm, cut the fudge into pieces. You can easily remove the fudge from the pan by grabbing the ends of the parchment paper and lifting the squares out.