Line a 9x13-inch baking pan with parchment paper.

Melt the butter in a heavy saucepan over medium heat.

Add the granulated sugar and evaporated milk to the pot.

Stir until the mixture blends smoothly.

Bring the mixture to a full boil.

Stir constantly for 5 minutes while the mixture boils.

Remove the pot from the heat.

Add the semi-sweet chocolate chips.

Stir until the chocolate chips melt completely.

Add the marshmallow creme to the pot.

Stir until the mixture becomes smooth and glossy.

Add the vanilla extract.

Fold in the chopped walnuts if using.

Pour the fudge mixture into the lined baking pan.

Smooth the top with a spatula.

Let the fudge cool completely at room temperature.

Lift the set fudge from the pan using the parchment.

Cut into 36 small squares.