2 pkg (11-oz each) classic white baking chips
3/4 cup bittersweet chocolate chips, divided
nonstick cooking oil spray
1 can (14-oz) sweetened condensed milk
Line an 8-inch square baking pan with foil, allowing foil to extend over 2 ends. Coat foil with the cooking spray; set aside.
In a large saucepan combine the white baking chips with the sweetened condensed milk. Heat and stir over very low heat until melted and smooth. Watch very carefully to avoid scorching! Quickly pour half the white mixture into the prepared pan. Sprinkle with 1/2 cup of the chocolate chips; top with the remaining white mixture. Sprinkle the remaining 1/4 cup of chocolate chips over the top.
Allow to stand 1 minute for the chips to soften. Using a knife, gently swirl the mixtures together to give a marbled effect.
Cover and allow to stand at room temperature 2 hours or until firm.
Lift the cooled, firm fudge from the pan using the extended foil to lift. Place on a cutting board and cut into 1-inch pieces.
May be stored in an airtight container at room temperature for up to 2 days or refrigerated up to 2 weeks.