Cinco de Mayo Treat: Just 5 ingredients. My neighbors asked for seconds before anyone else got firsts.

This 5-ingredient slow cooker vintage key lime custard cake is the kind of dessert that disappears before you can blink—my neighbors were asking for seconds before everyone else had even been served. It’s inspired by old-fashioned baked custards and pudding cakes, but streamlined for a modern slow cooker and busy home cooks. Using bottled key lime juice gives you that distinctive floral-tart flavor without the hassle of juicing a mountain of tiny limes. The texture lands somewhere between a silky custard and a very soft cake, with a pale green hue and light caramelization around the edges that feels delightfully nostalgic and perfect for a Cinco de Mayo treat.

Serve this key lime custard cake slightly warm or at cool room temperature, scooped straight from the slow cooker into small bowls. It pairs beautifully with lightly sweetened whipped cream or a spoonful of vanilla yogurt to soften the tartness. Add a few fresh berries or thin slices of strawberry if you want color without complicating the recipe. For drinks, try a cup of coffee, a milky black tea, or a not-too-sweet sparkling water with lime. If you’re serving this at a Cinco de Mayo gathering, it sits nicely alongside savory tacos or enchiladas as a bright, creamy finish that won’t feel too heavy after a big meal.

Servings: 6–8

1 box (about 15.25 oz) yellow cake mix

Grease the slow cooker: Lightly coat the inside of a 4- to 6-quart slow cooker with a thin layer of softened butter or nonstick spray, making sure to cover the bottom and at least halfway up the sides. This helps the custard cake release easily and encourages that gentle caramelization around the edges.

Prepare the custard base: In a large mixing bowl, whisk together the sweetened condensed milk and key lime juice until completely smooth and slightly thickened. The acidity of the key lime juice will start to set the condensed milk a bit—that’s exactly what you want for a custard-like texture.

Add the eggs: Crack the eggs into a separate small bowl to check for shells, then add them to the lime-milk mixture. Whisk steadily but not aggressively until the eggs are fully incorporated and the mixture is silky and uniform. Avoid whipping in too much air; you’re aiming for a smooth custard, not a foamy batter.

Incorporate the melted butter: Slowly drizzle the melted butter into the custard mixture while whisking. The butter adds richness and helps create that vintage, pudding-cake mouthfeel. Whisk until you no longer see streaks of butter and the mixture looks glossy.

Fold in the cake mix: Sprinkle the dry yellow cake mix over the custard mixture. Using a spatula or wooden spoon, gently fold and stir until no dry pockets of cake mix remain. The batter will be fairly loose compared to a traditional cake batter—this is what creates the layered custard-cake effect as it cooks.

Create a swirl on top: Pour the batter into the greased slow cooker and gently tap the base on the counter to release any large air bubbles. To mimic a delicate swirl pattern on top, drag the tip of a butter knife or a skewer lightly across the surface in loose, overlapping curves. Don’t overwork it; a few simple swirls will show after cooking.

Slow cook the custard cake: Cover the slow cooker with its lid, placing a clean kitchen towel under the lid if you’d like to catch condensation and keep the surface smoother. Cook on LOW for 2.5 to 3.5 hours, or until the edges are lightly golden and set, the center jiggles slightly like soft custard, and a thin knife inserted near the edge comes out mostly clean. Avoid opening the lid during the first 2 hours so the custard sets properly.

Rest before serving: Turn off the slow cooker and remove the lid, keeping the appliance uncovered. Let the custard cake rest in the warm crock for 20 to 30 minutes. This resting time allows the center to finish setting gently and helps the top stay smooth, with pale green creaminess and light caramelization around the sides.

Serve from the slow cooker: Scoop the key lime custard cake directly from the slow cooker with a large spoon or ladle, making sure each portion has some of the soft center and the slightly firmer, caramelized edges. Serve warm or at room temperature. If you like, garnish each serving with a small dollop of whipped cream and a tiny wedge of lime or a few lime zest shavings for aroma.

Cool and store leftovers: Once the slow cooker insert has cooled to room temperature, cover it tightly or transfer leftovers to a shallow container. Refrigerate for up to 3 days. The texture will firm slightly when cold but remain creamy; you can enjoy it chilled or gently rewarmed in short bursts in the microwave.

To keep the recipe aligned with the five-ingredient promise, most of the variation happens in how you serve and handle the custard cake rather than adding new components. For a milder lime flavor, replace 1/4 cup of the key lime juice with whole milk or half-and-half; this softens the tartness and yields a slightly richer custard. If you prefer a more pronounced color, you can very lightly tint the batter with a drop or two of green food coloring, but the natural pale green is more in keeping with the vintage style. For a citrus twist, substitute half of the key lime juice with lemon juice for a key lime–lemon custard cake. If you need to use a different cake mix, white cake mix works similarly, giving a slightly lighter flavor and color. Food safety tips: Always use pasteurized eggs, and avoid tasting the batter once the eggs are added, since it contains raw egg. Cook on LOW, not on WARM, to ensure the custard reaches a safe temperature (at least 160°F/71°C in the center). If your slow cooker runs hot and the edges are browning too quickly while the center is still very loose, switch to the WARM setting for the last 30–45 minutes to prevent scorching. Refrigerate leftovers within 2 hours of finishing the resting period, and discard any that have been left at room temperature for more than 2 hours, especially in warm weather or at a crowded Cinco de Mayo party.