Spray a 9-inch square pan with cooking spray and then line with wax paper.
In a large microwave-safe bowl add the chocolate chips and vanilla chips. Microwave at 1-minute intervals for 2 minutes.
After the chips are softened stir in the sweetened condensed milk and microwave another minute and stir. Repeat until the chips are softened and can be stirred into the milk smoothly.
Pour fudge mixture into the prepared pan and place in the refrigerator to chill for about an hour or until firm.
After fudge is firm, lift the fudge out of the pan with the wax paper and cut into squares.
Store in an airtight container.