Combine the semi-sweet chocolate chips, sweetened condensed milk, and miniature marshmallows in a medium-size saucepan over medium heat. Stir frequently as the chocolate melts.

Add the creamy peanut butter and vanilla extract. Stir to melt and combine well.

Remove from the heat. Lightly grease a 10-inch square pan or line with parchment. Pour the fudge into the pan and spread with a spatula.

Chill in the refrigerator until firm, about 3 hours. Slice into 1-inch pieces and store in an airtight container in the refrigerator.