Line a 9" x 13" pan with foil or grease it well with butter.

In one small pan, combine chocolate chips and condensed milk. Heat on low, stirring frequently until melted.

Remove chocolate from heat and add vanilla extract, combine.

In another pan, add peanut butter and butter. Heat this one also on low, stirring often until melted.

Add sugar into peanut butter mixture, stir to combine. Remove it from heat.

Pour the chocolate mixture into the greased pan, pour the peanut butter mixture over the top. Pull a toothpick through the warm fudge if you want a marble effect.

Place in refrigerator until firm, about 2 hours. Cut and enjoy!