Lightly spray a 9 x 13 baking dish.: When you rub your fingers over a gently oiled surface you will notice a faint slip that helps the finished squares release cleanly, which matters for presentation. I prefer a light coat rather than heavy, because excess oil can pool and affect how the salad sets along the edges. A common mistake is using too much spray, which can create an oily ring on the bottom; wipe away any obvious globs before pouring in the mixture. This step sets you up for neat serving pieces and less stress when plating.
Dissolve both boxes of JELLO and unflavored gelatin in hot water in baking dish.: As the powders hit the hot water , steam rises and a fragrant citrus scent will begin to fill the kitchen, signaling proper dissolution. Stir continuously with a whisk to make sure there are no gritty bits, and watch the liquid become clear and shiny, an indication the gelatin has fully bloomed. If you stop stirring too soon, you risk small lumps that will stay in the final salad, so keep mixing until silky. Avoid pouring water that is only warm, as it will not dissolve the powders fully and can leave a grainy texture.
Stir in sweetened condensed milk, pineapple, cottage cheese and mayonnaise until thoroughly combined.: Once the gelatin base is smooth, adding the sweetened condensed milk mellows the tartness and creates a creamy sheen. As you fold in the pineapple , you will hear a slight glug from the fruit juice, and the cottage cheese curds will distribute into the glossy mixture, offering visual contrast. Use gentle strokes so the curds remain intact but distribute evenly; overmixing can break them down too much. A frequent error is adding cold dairy straight into very hot gelatin, which can shock the dairy; temper by stirring a little at a time so the temperature evens out.
Chill until firm, at least 2 hours.: Place the dish in the refrigerator and resist the urge to peek often, because jostling while it sets can create uneven pockets. After around 2 hours the surface should have a gentle bounce, and a toothpick will come out with only a tacky residue when inserted. I like to chill it longer if possible, as overnight resting improves sliceability. Mistakes here include removing it too early, which results in runny slices; be patient and wait for a firm set.
Sprinkle on lemon and lime zest if desired.: Fresh zests bring an immediate aromatic lift that contrasts with the sweet, creamy base. Use a microplane to grate only the colorful outer layer so you avoid bitter pith, and lightly dust the top just before serving to preserve fragrance. If you add zest too early it can disperse and lose its punch, so timing matters. A common slip is using too much zest, which can feel overpowering; a little goes a long way.
Cut into squares and serve cold.: When the salad is fully chilled, the edges should separate cleanly from the dish. Use a sharp knife warmed briefly under hot water and wiped dry for cleaner cuts, wiping the blade between slices to maintain neat edges. Serve chilled for the best mouthfeel, because warmer pieces become softer and lose structure. One pitfall is slicing with a cold, residue coated knife which drags the set; keep the blade clean and warm for tidy portions.