Butter a square pan and line with parchment paper so it hangs over two of the sides for easy removal. Set aside.
In a microwave-safe bowl, add chopped chocolate and sweetened condensed milk. Microwave on high for 1 minute. Remove from the microwave. Let sit for 1 minute, then stir to combine. If needed, heat an additional 30 seconds. Stir until chocolate is completely melted the mixture is smooth.
Stir in vanilla extract, sea salt, stir in sprinkles if using. Pour fudge into prepared pan. Sprinkle some more sprinkles on top.
Let fudge cool completely before cutting into 1-inch squares. Place it in the refrigerator for 1 hour to speed up the process.