Line a 9x13 inch pan with parchment paper, leaving overhang for easy removal. Set aside.
Combine the chocolate chips and butter in a microwave-safe bowl. Microwave in 30-second bursts, stirring each time, until the mixture is smooth and glossy—about 1.5 minutes total.
Stir in the sweetened condensed milk and vanilla extract until fully incorporated, creating a shiny, thick mixture.
Pour the chocolate mixture into the prepared pan. Use a spatula to spread it evenly across the surface, smoothing out the top. Gently tap the pan on the counter to settle the mixture and remove air bubbles.
If desired, sprinkle chopped nuts, sea salt, or sprinkles on top, pressing them lightly into the surface for better adhesion.
Refrigerate the pan for at least 2 hours, or until the fudge is firm and set. The surface should feel crackly when gently pressed.
Lift the fudge out of the pan using the parchment overhang, then transfer to a cutting board. Slice into squares or rectangles with a sharp knife.
Serve immediately or store in an airtight container in the fridge. For softer texture, let the fudge sit at room temperature for 10 minutes before serving.
