Prepare your baking dish by lining it with parchment paper for easy fudge removal later.
In your sauce pan, melt the butter over medium heat.
Add in the evaporated milk and sugar, and stir until well combined.
Bring to a boil, stirring constantly. Use your candy thermometer to watch until the mixture reaches 234° F. Undercooked and it won't set, overcooked and it will crystallize.
Once your mixture reaches temperature, remove it from the heat and stir in your chocolate chips, mixing until they're melted.
Add in your vanilla, stir again, and ensure it's thoroughly mixed.
Spread your fudge mixture in your baking dish as evenly as you can. Set it aside to cool for at least an hour until it sets before removing, cutting, and serving.