Line an 8x8-inch square baking pan with foil or parchment paper. Set aside.
Place the chocolate and butter into a large microwave-safe bowl, and heat in 30 second intervals, stirring after each one, until melted and smooth.
Stir in the condensed milk and vanilla until combined. The mixture will be very thick.
Spread the mixture into the prepared pan, and smooth out the top. Refrigerate for 2 - 4 hours or overnight until completely set.
Notes
Fudge can be stored, in an airtight container at room temperature for 1 week, or in the fridge for 2 - 3 weeks. It also freezes well for up to 2 months. Thaw overnight in the fridge before serving.