Line an 8×8-inch square baking pan with parchment paper, leaving an overhang on two sides to form a sling. Lightly grease the parchment with butter or nonstick spray for easy removal and clean slicing.

Zest the lemons first, then juice them. Measure out 2–3 tablespoons lemon zest and 1–2 tablespoons fresh lemon juice. Avoid the bitter white pith when zesting.

In a medium heavy-bottomed saucepan over low heat, combine the white chocolate chips, sweetened condensed milk, and unsalted butter. Stir constantly with a silicone spatula, scraping the bottom and sides to prevent scorching.

Continue cooking over low heat, stirring constantly, until the mixture is fully melted, thick, smooth, and glossy, with no visible chunks of chocolate. This usually takes 5–8 minutes. If it begins to bubble, reduce the heat.

Remove the saucepan from the heat. Stir in the lemon curd, 2 tablespoons lemon zest, 1 tablespoon lemon juice, 1 teaspoon vanilla extract, and fine sea salt until fully combined and smooth. Taste a small amount carefully; if you want a stronger tang, add up to 1 more tablespoon lemon juice. If the mixture becomes too loose, stir in a small handful of extra white chocolate chips off the heat until thickened.

If desired, add 2–4 drops of yellow food coloring to the warm fudge base and stir until the color is evenly distributed, adjusting to your preferred shade.

Scrape the lemon fudge mixture into the prepared pan and spread it into an even layer, smoothing the surface with a spatula. Tap the pan gently on the counter a few times to release any air bubbles.

In a small bowl, stir together the marshmallow creme, 1 teaspoon lemon zest, and 1 teaspoon vanilla extract until smooth and combined. If the marshmallow creme is very stiff, microwave it for about 10 seconds to loosen before mixing.

Drop spoonfuls of the marshmallow mixture all over the warm fudge. Use a butter knife or skewer to gently swirl it into the fudge in figure-eight motions, making 3–4 passes so you have distinct ribbons rather than fully mixing it in. If using, sprinkle crushed shortbread or graham crackers and/or lightly crushed mini meringue cookies over the top and gently press them in.

Cover the pan loosely with foil or plastic wrap (avoid touching the surface if possible) and refrigerate for at least 2–3 hours, or until the fudge is firm enough to cut. For very clean slices, chill 4 hours or overnight.

Use the parchment sling to lift the fudge out of the pan and place it on a cutting board. With a sharp knife, cut into 25 small squares (5×5 grid) or up to 36 mini squares. Wipe the knife with a warm, damp cloth between cuts for neat edges. Serve chilled or allow to soften slightly at room temperature before serving.