Line an 8x8-inch pan with parchment paper, leaving an overhang on two sides to act as handles.
In a medium saucepan, combine the sweetened condensed milk, white chocolate chips, molasses, and butter.
Cook over low-medium heat, stirring constantly, until the chocolate is fully melted and the mixture is smooth. Do not let it boil.
Remove the pan from the heat. Stir in the ground ginger, cinnamon, nutmeg, allspice, cloves, and vanilla extract until well combined.
Quickly pour the fudge mixture into the prepared pan and spread it into an even layer with a spatula.
Let the fudge set at room temperature for 3-4 hours, or in the refrigerator for about 2 hours, until firm.
Once firm, use the parchment handles to lift the fudge out of the pan. Cut it into small squares with a sharp knife.
For the optional drizzle, melt the 4 ounces of white chocolate in a microwave-safe bowl in 20-second intervals, stirring between each, until smooth.
Drizzle the melted chocolate over the fudge squares. Let the drizzle set for about 10-15 minutes before serving.