Line an 8x8-inch (20x20 cm) baking pan with parchment paper, leaving overhang on two sides to form handles. Lightly spray the parchment with cooking spray if needed. A 9x9-inch pan can be used for slightly thinner fudge.

In a small bowl, stir together the ground ginger, cinnamon, nutmeg, cloves, and fine sea salt until well combined. Set aside.

In a medium, heavy-bottomed saucepan, combine the white chocolate chips, sweetened condensed milk, and unsalted butter. Set over low heat and stir often with a silicone spatula or wooden spoon until the mixture is completely smooth and glossy, with no visible chocolate pieces. Keep the heat low to prevent scorching; if small bits remain, remove from heat and keep stirring so residual heat finishes melting.

Remove the pan from the heat. Immediately stir in the molasses and vanilla extract until incorporated; the mixture may thicken slightly. Sprinkle in the prepared spice mixture and stir very thoroughly, scraping the bottom and sides of the pan so the spices are evenly distributed. Taste a small dab carefully and adjust with a pinch more ginger or cinnamon if you want a stronger gingerbread flavor.

If using chopped nuts or crushed gingersnap cookies inside the fudge, gently fold them into the warm mixture now, reserving some for topping if desired. Avoid adding too many mix-ins so the fudge stays easy to slice.

Pour the fudge mixture into the prepared pan, scraping the saucepan well. Smooth the top with a spatula, nudging the fudge into the corners and leveling the surface. Work fairly quickly, as the fudge begins to firm as it cools.

While the fudge is still warm and soft, sprinkle the surface with crushed gingersnap cookies, chopped nuts, and holiday sprinkles as desired, then lightly press them into the fudge with clean hands or the back of a spoon. Optionally dust very lightly with ground cinnamon or cocoa powder for a bakery-style finish.

Refrigerate the pan of fudge for 2–3 hours, or until fully set. For easiest slicing or make-ahead prep, chilling overnight works very well.

Once firm, lift the slab of fudge out of the pan using the parchment overhang and place it on a cutting board. Using a sharp knife, cut into about 1-inch squares (or your preferred size), wiping the knife with a warm, damp cloth between cuts for clean edges. Serve slightly chilled or let sit at room temperature for 5–10 minutes for a softer bite.