Line an 8×8-inch (20×20 cm) square baking pan with parchment paper, leaving an overhang on two sides to use as handles. Lightly mist the parchment with nonstick spray or brush with a small amount of melted butter.

In a large microwave-safe mixing bowl, add the white chocolate chips, sweetened condensed milk, and butter pieces. Distribute them evenly in the bowl, but do not stir yet.

Microwave the mixture on 50% power for 60 seconds. Remove and stir slowly, scraping the bottom and sides. Return to the microwave and heat in 20–30 second bursts at 50% power, stirring thoroughly after each burst, until the mixture is mostly melted and smooth. Stop when only a few small lumps remain; the residual heat will finish melting them.

Continue stirring the warm mixture for another 20–30 seconds until it is thick, glossy, and completely smooth. If it appears grainy, microwave for an additional 10 seconds at 50% power and stir again.

Stir in the bubblegum flavoring or extract and the fine sea salt. Taste a tiny amount (caution, it will be warm) and add up to an additional 1/2 teaspoon bubblegum flavoring if desired. Add pink gel food coloring one drop at a time, stirring well after each drop, until you reach your preferred shade of bubblegum pink.

Let the mixture cool for 2–3 minutes so any mix-ins don’t completely melt. Gently fold in mini marshmallows, chopped bubblegum candies, and/or extra white chocolate chips or colorful candy pieces, if using, until evenly distributed.

Quickly scrape the fudge mixture into the prepared pan. Use a rubber spatula or small offset spatula to spread it evenly into the corners and smooth the top. Work fairly quickly, as the fudge will begin to firm as it cools.

While the surface is still soft, sprinkle the top with rainbow sprinkles or other decorative toppings. Press them down very lightly so they adhere to the fudge.

Place the pan in the refrigerator and chill for at least 2 hours, or until the fudge is firm enough to slice cleanly. For the neatest squares, chill 3–4 hours or overnight.

Use the parchment overhang to lift the chilled fudge from the pan onto a cutting board. With a sharp knife, cut into about 36 1-inch squares, wiping the knife with a warm, damp cloth and drying between cuts for clean edges. Serve chilled or at cool room temperature.