Old-Fashioned Chocolate Mayonnaise Cake (Super Moist)

It sounds strange until you taste it: mayonnaise — the same jar from the sandwich aisle — makes one of the moistest chocolate cakes you will ever bake. This old-fashioned chocolate mayonnaise cake is a Depression-era classic born from rationing, when eggs and oil were scarce and home bakers got creative.

The mayonnaise does the quiet work here. It is simply eggs and oil emulsified, so it brings richness, moisture, and a velvety crumb to the cake without any detectable flavor. The result is a deeply chocolatey, almost fudgy sheet cake with a tender texture that stays soft for days.

Finished with a simple glossy chocolate frosting right in the pan, this is the kind of cake that shows up at potlucks and disappears before the ice cream melts.

Why You’ll Love This Recipe

• Unbelievably moist — mayonnaise keeps the crumb tender for days
• No eggs or oil to measure — the mayo replaces both in one step
• Deep chocolate flavor — hot water blooms the cocoa for extra intensity
• One-pan easy — mix, bake, and frost right in the same 9×13 pan
• Crowd-pleaser — a nostalgic classic that always gets recipe requests

Ingredients

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For the cake:

• 2 cups (250 g) all-purpose flour
• 1 cup (85 g) unsweetened cocoa powder
• 2 cups (400 g) granulated sugar
• 2 teaspoons baking soda
• 1 teaspoon fine sea salt
• 1 cup (225 g) full-fat mayonnaise
• 1 cup (240 ml) hot water
• 1 teaspoon vanilla extract

For the frosting:

• 1/2 cup (115 g) unsalted butter, softened
• 1/3 cup (30 g) unsweetened cocoa powder
• 3 cups (360 g) powdered sugar
• 1/4 cup (60 ml) milk, plus more as needed
• 1 teaspoon vanilla extract
• Pinch of fine sea salt

Instructions

1. Heat the oven. Heat to 350°F (175°C). Grease and flour a 9×13-inch baking pan, or line it with parchment paper.
2. Whisk the dry ingredients. In a large bowl, whisk together the flour, cocoa powder, sugar, baking soda, and salt until no lumps remain.

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3. Add the wet ingredients. Add the mayonnaise, hot water, and vanilla. Beat with a whisk or electric mixer on medium speed for about 2 minutes, until the batter is smooth and glossy.
4. Bake. Pour into the prepared pan and smooth the top. Bake 30–35 minutes, until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).

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5. Cool. Let the cake cool in the pan on a wire rack for at least 30 minutes. Frost while still slightly warm for an extra-fudgy finish, or cool completely for neat slices.
6. Make the frosting. Beat the butter until creamy. Sift in the cocoa powder, then beat in the powdered sugar, milk, vanilla, and salt until smooth and spreadable, adding milk a spoonful at a time as needed.
7. Frost and serve. Spread the frosting evenly over the cake right in the pan. Slice into 15 squares and serve.

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Tips for Success

• Use full-fat mayonnaise. Light or low-fat versions contain more water and stabilizers and will not give the same rich crumb.
• Sift the cocoa powder. Cocoa clumps stubbornly — sifting guarantees a lump-free batter and smooth frosting.
• Do not overbake. The cake continues cooking in the hot pan; pull it when the toothpick shows moist crumbs, not clean.
• Hot water matters. It blooms the cocoa, deepening the chocolate flavor and dissolving the sugar for a smoother batter.
• Frost generously. This cake can handle a thick layer of frosting — it balances the deep chocolate crumb.

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Variations

• Mocha mayo cake: replace the hot water with hot brewed coffee for a subtle coffee depth.
• Texas sheet cake style: pour a warm pourable chocolate glaze over the hot cake instead of buttercream.
• Layer cake: bake in two 9-inch rounds for 25–30 minutes and stack with frosting between the layers.
• Peanut butter twist: add 1/2 cup creamy peanut butter to the frosting for a chocolate-peanut butter finish.

Storage

Cover the frosted cake tightly and store at room temperature for up to 3 days, or refrigerate for up to 5 days. Bring refrigerated slices to room temperature before serving for the best texture. The unfrosted cake freezes well for up to 2 months — wrap the whole pan in two layers of plastic wrap.

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FAQ

Can you taste the mayonnaise? No. The mayonnaise melts into the batter and tastes like nothing but moisture — it is simply eggs and oil, which most cake recipes contain anyway.

Can I use Miracle Whip instead? It is not recommended — Miracle Whip is sweeter and tangier, which changes the flavor. Use real, full-fat mayonnaise.

Why did my cake sink in the middle? Usually from opening the oven door early, underbaking, or expired baking soda. Check that your baking soda is fresh and keep the door closed for the first 25 minutes.

Can I make this dairy-free? Use a dairy-free mayonnaise, replace the butter in the frosting with a plant-based butter, and use a non-dairy milk. The cake itself contains no other dairy.

Recipe Card

• Prep time: 15 minutes
• Cook time: 35 minutes
• Total time: 50 minutes
• Servings: 15
• Estimated calories per serving: ~420 per square

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