Line an 8x8-inch (20x20 cm) pan with parchment paper or aluminum foil, leaving overhang on two sides to form handles. Lightly grease the parchment or foil with butter or nonstick spray for easier removal.
In a medium microwave-safe bowl, add the semi-sweet chocolate chips, sweetened condensed milk, and butter pieces. Have the vanilla and salt measured and ready nearby.
3 cups semi-sweet chocolate chips, 14 oz can sweetened condensed milk, 3 tablespoons unsalted butter
Microwave the chocolate mixture on medium power (50–70%) for 60 seconds. Remove the bowl and stir well. Return to the microwave and heat in 20–30 second intervals, stirring after each burst, until the chocolate is mostly melted and the mixture is thick and glossy. Avoid using full power to prevent scorching.
3 cups semi-sweet chocolate chips, 14 oz can sweetened condensed milk, 3 tablespoons unsalted butter
When the chocolate is about 80–90% melted, stop microwaving and continue stirring until completely smooth; the residual heat will finish melting any remaining bits.
3 cups semi-sweet chocolate chips, 14 oz can sweetened condensed milk, 3 tablespoons unsalted butter
While the mixture is still warm and spreadable, stir in the vanilla extract and fine sea salt until well combined. If using espresso powder or chopped nuts, fold them in now. If the mixture becomes too stiff to stir, microwave for an additional 10–15 seconds and stir again.
1 teaspoon pure vanilla extract, 1/4 teaspoon fine sea salt, 1/2-1 cup chopped nuts, 1 teaspoon espresso powder
Scrape the fudge mixture into the prepared pan. Use a silicone spatula or the back of a spoon to spread it evenly and press it into the corners. Tap the pan gently on the counter a few times to help smooth the top.
While the fudge is still soft on top, sprinkle with mini chocolate chips or white chocolate chips, crushed peppermint candies, chopped nuts, or flaky sea salt, if desired. Gently press the toppings into the surface so they adhere.
1/2-1 cup chopped nuts, 2-3 tablespoons mini chocolate chips or white chocolate chips, 1-2 tablespoons crushed peppermint candies, flaky sea salt
Transfer the pan to the refrigerator and chill for at least 2 hours, or until the fudge is firm enough to slice. For a faster set, place in the freezer and begin checking after 45–60 minutes, making sure it is firm but not icy.
Use the parchment or foil handles to lift the fudge from the pan onto a cutting board. With a sharp knife, cut into small squares, about 1 to 1½ inches each. Wipe the knife with a warm, damp cloth between cuts for clean edges.
Serve the microwave fudge at room temperature for the creamiest texture. Enjoy as part of dessert trays, gift boxes, or as a quick sweet treat.