Preheat oven to 350°F.: When you open the oven you should feel a gentle wave of warm air, and the interior should be evenly heated so the bars bake uniformly. Preheating ensures the exterior sets quickly while the center stays tender. If you skip preheating, the texture becomes uneven and the center may stay underbaked.
Line a 9×9 inch baking pan with foil. Spray with cooking spray and set aside.: Lining the pan with foil and greasing it prevents sticking and makes it easy to lift the whole slab out for slicing. Smooth the foil carefully to avoid tears which can cause leaking. If the foil is wrinkled, the bars can bake unevenly at the creases, so press it flat.
Beat butter with a mixer until soft and fluffy.: At this stage the butter should appear paler and slightly aerated with soft peaks of motion in the bowl, which helps distribute fat for a tender crumb. Use medium speed and scrape the bowl to ensure even texture. Overbeating can incorporate too much air, leading to cracks, while underbeating leaves lumps.
Add in cake mix, egg, lime juice and zest and continue beating until combined.: The mixture will transform into a cohesive, slightly sticky dough that smells of strawberry and bright lime . Mixing until just combined avoids developing a tough texture. Mixing too long will make the bars dense rather than tender.
Press about two-thirds of the cake mixture onto the bottom of the foil-lined baking pan.: Use your palms or the back of a spoon to create an even base layer, pressing firmly but not compacting to death, aiming for a uniform thickness. The base should be visibly even so the gooey layers sit flat. Pressing too thin will leave gaps, pressing too hard will make the final bars dense.
Sprinkle white chocolate chips over the top.: Scatter the white chocolate chips evenly so they distribute melting throughout the bars, creating pockets of creamy sweetness. You should see little mounds of chips dotting the surface. Piling them in one spot will cause uneven melting and a heavy center.
Drizzle the sweetened condensed milk over the top of the chocolate.: The sweetened condensed milk should fall in a smooth ribbon and pool slightly around the chips, forming a glossy layer that will caramelize some during baking. Aim for even coverage so the goo is consistent. If too much pools in one area, the center may become overly wet.
Crumble the remaining cake mixture over the top of the sweetened condensed milk, trying to spread it out as much as possible. Press lightly.: The top will be a broken, patchy crust that lets the goo peek through, which creates a contrast in texture when baked. Pressing lightly helps the crumbs adhere without sealing them. Pressing too hard will bury the goo and prevent that signature marbled effect.
Bake for 29-32 minutes, or until edges start to brown.: You want to watch for a gentle golden color along the edges and a set but still soft center, the scent should be warmly buttery and fragrant with strawberry . Overbaking will dry them out, while underbaking leaves the interior too loose. If your oven runs hot, check at the earlier time.
Let bars cool completely before cutting.: Cooling allows the goo to set slightly so the bars hold their shape, and the aroma will calm into a richer, more integrated scent. If you cut while warm, the bars will crumble and the goo will ooze. Patience here yields neater squares.
Top with additional lime zest if desired.: Fresh lime zest on top adds a final aromatic burst and a bright visual touch. Grate finely so it blends into each bite rather than creating large bits. Adding too much zest can impart bitterness, so use sparingly.
Store in an airtight container for up to a week.: Stored properly the bars keep their texture, with the edges softening and the center staying pleasantly gooey. I like stacking parchment between layers to prevent sticking. Leaving them exposed will dry them out, so airtight is key.
