In a large mixing bowl, combine the chocolate, sweetened condensed milk, and extract. Microwave on high for 1-1.5 minutes, in 30-second intervals. Stir in between. Or you can melt this in a glass mixing bowl set over a pot of simmering water (double boiler).
Once the mixture is melted and smooth, pour into a square baking dish lined with parchment paper. Top with salt, sprinkles, or nuts if using. Allow to fully set before cutting into squares. If you want to speed up this process, you can pop the fudge in the refrigerator or freezer.
You can swap the vanilla extract for other flavors like peppermint or almond to change up the fudge's flavor.