Line an 8x8-inch baking dish with parchment paper.

In a large, heavy saucepan over medium-high heat, melt the butter.

Cook the butter until it turns a rich, golden brown with a nutty aroma, stirring occasionally to prevent burning.

Reduce heat to medium and add sugar and evaporated milk. Bring to a boil and cook until the mixture reaches 234°F-238°F on a candy thermometer.

Remove from heat at 234°F and stir in white chocolate chips until mostly melted.

Fold in marshmallow fluff and vanilla extract.

Transfer the fudge to the prepared pan and let it cool for at least 2 hours before cutting into squares.