Line an 8×8-inch (20×20 cm) baking pan with parchment paper, leaving overhang on two sides to form handles. Lightly spray the parchment with nonstick spray if needed to prevent sticking.

Measure out the cotton candy flavoring and have the pink and blue gel food colorings and pastel sprinkles ready near your workspace so you can work quickly once the fudge base is warm.

In a large microwave-safe bowl, combine the white chocolate chips, sweetened condensed milk, and butter. Microwave on 50% power for 60 seconds, then stir well. Continue microwaving in 20–30 second bursts at 50% power, stirring after each, until the chocolate is mostly melted with just a few small lumps remaining.

Remove the bowl from the microwave and stir until the mixture is completely smooth, thick, and glossy. If small chunks of chocolate remain, microwave once more for 10–15 seconds at 50% power and stir again.

Stir in the vanilla extract and fine sea salt. Add 1 teaspoon of cotton candy flavoring, mix well, and taste a tiny bit carefully (it will be warm). If desired, add up to an additional 1/2 teaspoon to intensify the flavor, keeping in mind it will mellow slightly as the fudge chills.

Quickly divide the warm fudge mixture evenly between two bowls. You will tint one portion pink and the other blue to create the swirl.

Add 1–2 drops of pink gel food coloring to one bowl and stir until the color is fully blended and pastel. Add an extra small drop if needed, but keep the color soft rather than neon.

Add 1–2 drops of blue gel food coloring to the second bowl and stir until evenly colored, aiming for a light cotton candy blue shade.

Working fairly quickly before the fudge thickens, spoon alternating dollops of pink and blue fudge into the prepared pan, creating a patchwork or checkerboard pattern. Use a small offset spatula or the back of a spoon to gently nudge the dollops together and level the surface.

With a butter knife or skewer, gently drag through the fudge in wavy lines in one direction, then in the perpendicular direction to lightly swirl the colors. Make only a few passes so the pink and blue remain distinct and do not blend into a single color.

While the fudge is still soft, sprinkle the top evenly with pastel sprinkles or nonpareils if using. Press the sprinkles very lightly with clean hands or the back of a spoon so they adhere.

Cover the pan loosely with plastic wrap or foil and refrigerate for at least 2 hours, or 3–4 hours or overnight for the firmest texture and cleanest cuts, until the fudge is completely set.

Use the parchment handles to lift the chilled fudge from the pan onto a cutting board. With a sharp knife (wiping it with a warm, damp cloth between cuts), slice into about 36 small 1-inch squares or 25 larger squares.

If desired, top each square with a tiny tuft of loose cotton candy just before serving. Do not add cotton candy in advance, as it will melt on contact with moisture.