Line an 8×8-inch baking pan with foil, leaving an overhang on two sides; set the pan aside.

Put 3/4 cup butter in a large microwave-safe bowl. Microwave on HIGH for 1 minute, then stir until the butter is completely melted.

Add 3 cups granulated sugar and 2/3 cup evaporated milk to the bowl and stir until combined.

Microwave the mixture on HIGH for 3 minutes. Carefully remove the bowl and stir.

Microwave on HIGH for 2 minutes. Carefully remove and stir.

Microwave on HIGH for 3 minutes. Carefully remove and stir. (The mixture will begin to bubble and expand.)

Microwave on HIGH for 2 1/2 minutes. Carefully remove and stir. Be careful—the bowl and mixture will be very hot, and the mixture can bubble and rise to twice its original volume; use a large microwave-safe bowl.

Let the hot mixture sit for 2 minutes to settle slightly.

Stir in 12 oz semi-sweet chocolate chips until they are fully melted and the mixture is smooth.

Add 7 oz marshmallow creme and stir until the fudge is fully combined and smooth.

Pour the fudge into the prepared foil-lined pan and spread it evenly.

Let the fudge cool completely at room temperature until set.

Use the foil overhang to lift the set fudge from the pan, peel off the foil, and cut into squares. Store at room temperature.