Line a 9" x 9" pan with foil or parchment paper and coat with nonstick cooking spray.
Melt butter.
Add chocolate chips to the melted butter and microwave for one minute and mix. If the chips aren't completely melted, continue to microwave at 30 second increments.
Add sweetened condensed milk and vanilla and gently stir.
Add walnuts or other topping, as desired.
Pour into pan and refrigerate about an hour. Cut into pieces and enjoy!