PREPARE: Line a 9x13-inch pan with parchment paper, leaving an overhang (foil also works). If using foil, lightly spritz with cooking spray. Set aside.
WHITE CHOCOLATE: Add all the white chocolate chips and 3/4 cup peanut butter to a large microwave-safe bowl. Microwave for 30 seconds and stir for 20 seconds. Return to the microwave and microwave in bursts of 15 seconds, stirring in between each burst for 10 seconds, until chocolate is melted and smooth. Don't microwave once the chips are mostly melted; just continue to stir until smooth. Pour into the prepared 9x13 pan.
MILK AND SEMI-SWEET CHOCOLATE: Put all the semi-sweet and milk chocolate chips to a large bowl along with remaining 2 tablespoons peanut butter. Microwave for 30 seconds and stir for 20 seconds. Return to the microwave and microwave in bursts of 15 seconds, stirring in between each burst for 10 seconds, until chocolate is melted and smooth. Don't microwave once the chips are mostly melted; just continue to stir until smooth. Drizzle chocolate layer over white chocolate layer and use a table knife to swirl the layers together. Place in the fridge for 30 minutes to an hour, until chilled and firm.
SERVE: Using the overhang, lift the block of fudge from the pan. Cut into small pieces and serve. Store leftovers in the fridge (they get melty at room temperature). These bars store in the fridge for up to one week.
Notes
Note 1: Chocolate: When it's the primary ingredient in a recipe, quality really matters. And that's the case in this recipe. Try to get high-quality chocolate (white, milk and semi-sweet). Not only will the flavor be better, but the texture of the Tiger Butter Fudge will improve. Poor-grade chocolate often has lots of fillers, including was. You sure don't want waxy fudge that tastes off!