Grandma Jo swore you could bring this to any potluck and come home with an empty dish.

Serve warm, scooped into bowls, topped with whipped cream or vanilla ice cream. If you reserved lime zest, sprinkle a little over each serving just before bringing it to the table or potluck.

To transport to a potluck, let the dish cool until just warm, then set it on a rimmed baking sheet for easier carrying. Cover loosely with foil so the topping stays a bit crisp and the buttery crumbs don’t steam too much.

For a slightly richer flavor, you can add 2–3 tablespoons of softened cream cheese in small dollops over the key lime pie filling before adding the cake mix; it gives pockets of creamy tang that still fit right in at a potluck. If you like a stronger lime flavor, stir an extra tablespoon of lime juice into the pie filling or add more lime zest on top after baking. A handful of finely chopped pecans or walnuts sprinkled over the cake mix before the butter will give the top a nutty crunch, which is very much in line with old Midwestern dessert bars. For smaller gatherings, halve the recipe and bake it in an 8×8 or 9×9-inch dish, checking for doneness 5–10 minutes earlier. If you don’t have key lime pie filling, you can use regular lemon pie filling or a mix of lemon and lime pie fillings for a cousin to this dish that still disappears at potlucks. Leftovers reheat nicely in the oven at 300°F for about 10 minutes, or in the microwave in short bursts, though the top is crispiest the day it’s baked. To keep the spirit of Grandma Jo’s magic empty dish, always bring a serving spoon and set the cake near the front of the dessert table—once folks get a look at that golden, buttery top and gooey edges, it won’t last long.