1. Line an 8-inch square baking pan with parchment paper, leaving overhang on the sides for easy lifting.

2. Add semi-sweet chocolate chips, sweetened condensed milk, and butter to a saucepan over low heat.

3. Stir gently until the chocolate melts and the mixture becomes smooth and glossy.

4. Remove from heat and stir in the vanilla extract.

5. Pour the fudge mixture into the prepared pan and spread it evenly.

6. Sprinkle white chocolate chips over the warm fudge and gently swirl them into the top.

7. Add Christmas sprinkles, crushed candy canes, mini marshmallows, chopped nuts, and flaky sea salt.

8. Press the toppings lightly into the fudge so they stick.

9. Refrigerate for at least 2 hours, or until fully firm.

10. Lift the fudge from the pan, slice into small squares, and serve chilled or at cool room temperature.