Optional mix-ins: 1/2 cup roasted nuts, candies, cookies, or fruit
Line an 8x8 baking dish with foil and set aside.
Add chocolate chips, sweetened condensed milk, salt, and espresso powder (if using) to a medium heavy-bottomed saucepan over medium-low heat. Melt the chocolate, stirring frequently. Remove from heat and stir in vanilla extract. Stir in nuts, candies, fruit, etc. if using.
Add chocolate chips and sweetened condensed milk to a large microwave-safe bowl. Heat at 30 second increments, stirring after each, until the fudge mixture is melted, smooth, and creamy.
Pour into prepared baking dish and spread with an offset spatula or a rubber scraper if needed. Cover with plastic wrap and refrigerate for 2 hours to set. Remove from refrigerator and remove from baking dish. Cut into 24 or 48 individual squares.
Store in layers with waxed paper or parchment dividing in an airtight container on the counter for up to 7 days, in the refrigerator for 3 weeks, or the freezer for up to 3 months.
To Make Ahead. Chocolate fudge is the perfect make-ahead treat or dessert. They keep well in an airtight container in a cool place for up to a week or in the refrigerator for up to 3 weeks.To Store. You can store fudge in individual squares or as a whole block of fudge. If storing as individual pieces, place waxed paper between the layers of fudge in an airtight container. If storing the whole block of fudge, wrap the fudge well with plastic wrap. Store in a cool place for up to a week or in the refrigerator for up to 3 weeks.To Freeze. Chocolate fudge freezes beautifully. Simply allow the fudge to harden in the refrigerator as instructed in the recipe. Then, either cut the fudge into individual pieces or freeze as a whole block of fudge. To freeze already cut into piece, place waxed paper between the layers of fudge in an airtight, freezer-safe container. To freeze a whole block of fudge, wrap well with plastic wrap and then wrap well with foil. Freeze for up to 3 months. To serve, remove from the freezer and allow to thaw overnight in a cool place.