Prep the pan. Line an 8-inch (20 cm) square baking pan with parchment, leaving overhang on two sides for easy lifting. Lightly mist with nonstick spray if you like.

Set aside some candies. Reserve about 1/3 cup of your mix-ins for sprinkling on top at the end. Lightly crush any larger candies (like mini eggs) so they slice more easily.

Melt the base gently. In a medium, heavy-bottomed saucepan, combine white chocolate chips, sweetened condensed milk, and butter. Warm over low heat, stirring constantly, until about 75% melted.

Finish melting off heat. Remove from heat and keep stirring until completely smooth. This prevents scorching and keeps the fudge silky.

Add flavor. Stir in vanilla and salt. Taste a tiny bit (careful, it’s warm) and adjust salt if needed. The mixture should taste sweet with a light, balanced finish.

Fold in most of the candies. Let the mixture cool for 2–3 minutes so candy coatings don’t melt. Fold in the remaining candies and any optional add-ins.

Spread and top. Scrape the mixture into the prepared pan. Smooth the surface with an offset spatula. Sprinkle the reserved candies evenly over the top and gently press them in.

Chill to set. Refrigerate uncovered for 2–3 hours, or until firm. For clean edges, chill at least 4 hours or overnight.

Slice. Lift the slab out using the parchment. Warm a large knife under hot water, wipe it dry, and cut into 25–36 small squares. Clean and re-warm the knife between cuts for neat pieces.

Serve or gift. Enjoy right away, or pack into airtight containers. Add a note with the date and storage tips if gifting.