Prepare an 8x8 inch baking pan by lining it with parchment paper. Make sure the parchment paper hangs over the edges to easily lift the fudge out later.
In a heavy-bottomed saucepan, combine the sugar, butter, milk, and salt. Cook over medium heat, stirring constantly, until the sugar is dissolved and the butter is melted.
Bring the mixture to a boil and cook, stirring occasionally, until the mixture reaches 235°F (113°C) on a candy thermometer. This may take 5-7 minutes.
Remove from heat and immediately add the chocolate chips and vanilla extract. Stir until the chocolate is completely melted and the mixture is smooth.
Pour the fudge into the prepared pan and spread evenly. Let it cool completely at room temperature, then refrigerate for at least 2 hours to set.
Once the fudge is firm, lift it out of the pan using the parchment paper overhang. Cut into squares and enjoy!
Notes
For a richer flavor, use dark chocolate chips instead of semi-sweet. You can also add chopped nuts or dried fruit to the fudge for extra texture and flavor.