Quick Recipe Version (TL;DR)
Quick Ingredients
- All-purpose flour 2 cups (240 g)
- Granulated sugar 1 1/2 cups (300 g)
- Natural unsweetened cocoa powder 1/2 cup (45 g)
- Baking soda 2 tsp; baking powder 1 tsp; fine salt 1/2 tsp
- Full-fat mayonnaise 1 cup (240 g)
- Cool water 1 cup (240 ml)
- Vanilla extract 2 tsp
- Powdered sugar 1 1/4 cups (150 g)
- Hot espresso 3 tbsp + hot water 1 tbsp
- Vanilla extract 1 tsp; pinch fine salt
- Rainbow sprinkles 2–3 tbsp
Do This
- 1) Heat oven to 350°F (175°C). Grease a 9×13-inch pan and line with parchment.
- 2) Whisk flour, sugar, cocoa, baking soda, baking powder, and salt. Add mayonnaise, water, and vanilla; whisk until smooth.
- 3) Pour into pan. Bake 28 minutes, until a toothpick at center has moist crumbs.
- 4) Cool in pan on a rack 20 minutes.
- 5) Whisk powdered sugar, espresso, hot water, vanilla, and salt until thin and glossy.
- 6) Pour glaze over warm cake; spread thinly. Add sprinkles; let set 15 minutes, then slice.
Why You’ll Love This Recipe
- Ultra-moist, tender crumb thanks to mayonnaise (no separate eggs or butter to measure).
- One-bowl batter: minimal dishes, maximum chocolate flavor.
- Thin espresso glaze adds café-style shine and subtle mocha notes without overpowering the cake.
- Stays fresh and soft for days—perfect for parties, potlucks, and birthdays.
Grocery List
- Produce: None
- Dairy: None
- Pantry: All-purpose flour, granulated sugar, natural cocoa powder, baking soda, baking powder, fine salt, full-fat mayonnaise, vanilla extract, powdered sugar, espresso (or instant espresso), rainbow sprinkles, parchment paper.
Full Ingredients
Chocolate Mayonnaise Sheet Cake (9×13-inch)
- All-purpose flour: 2 cups (240 g)
- Granulated sugar: 1 1/2 cups (300 g)
- Natural unsweetened cocoa powder: 1/2 cup (45 g)
- Baking soda: 2 teaspoons
- Baking powder: 1 teaspoon
- Fine sea salt: 1/2 teaspoon
- Full-fat mayonnaise: 1 cup (240 g)
- Cool water: 1 cup (240 ml)
- Vanilla extract: 2 teaspoons
Thin Espresso Glaze
- Powdered sugar: 1 1/4 cups (150 g)
- Freshly brewed hot espresso: 3 tablespoons
- Hot water: 1 tablespoon
- Vanilla extract: 1 teaspoon
- Fine sea salt: a pinch
Finish
- Rainbow sprinkles (jimmies or confetti): 2–3 tablespoons
Step-by-Step Instructions
Step 1: Heat the oven and prepare the pan
Set a rack in the center of the oven and preheat to 350°F (175°C). Lightly grease a 9×13-inch metal baking pan with neutral oil or nonstick spray. Line the bottom with a parchment sling (two strips that overhang the long sides) and lightly grease the parchment. This guarantees a clean release and tidy slices.
Step 2: Whisk the dry ingredients
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until the mixture looks uniform and no cocoa streaks remain. Whisking aerates and helps prevent lumps in the one-bowl batter.
Step 3: Add wet ingredients and make the batter
Add the mayonnaise, cool water, and vanilla directly to the bowl. Whisk gently at first, then more vigorously until the batter is smooth and glossy, about 45–60 seconds. Stop as soon as there are no dry pockets; overmixing can toughen the crumb.
Step 4: Bake
Pour the batter into the prepared pan and smooth the top. Bake for 28 minutes, or until the top springs back lightly and a toothpick inserted near the center comes out with a few moist crumbs. If your oven runs cool, you may need up to 32 minutes; begin checking at 26 minutes to be safe.
Step 5: Cool briefly and mix the glaze
Transfer the pan to a wire rack and cool the cake for exactly 20 minutes. Meanwhile, in a small bowl, whisk the powdered sugar, hot espresso, hot water, vanilla, and a pinch of salt until the glaze is fluid and pourable with the consistency of heavy cream. If needed, add 1/2 teaspoon hot water at a time to thin.
Step 6: Glaze, sprinkle, and set
While the cake is still warm but no longer hot, pour the glaze over the surface and nudge it to the edges with an offset spatula for a thin, even coat. Immediately shower with rainbow sprinkles. Let the glaze set for 15 minutes, then lift the cake out by the parchment and slice into 16 squares.
Pro Tips
- Weigh your flour for best texture (240 g). Too much flour = dry cake; the mayonnaise can’t fully rescue it.
- Use natural unsweetened cocoa powder for classic chocolate flavor that plays nicely with baking soda.
- Full-fat mayonnaise is key. Light mayo yields a less tender crumb and muted flavor.
- Glaze the cake while warm so it forms a thin, shiny layer that lightly soaks into the top.
- For clean slices, chill the glazed cake 20–30 minutes, then cut with a warm, dry knife.
Variations
- Mocha Boost: Replace the 1 cup water in the batter with strong brewed coffee, cooled.
- Double Chocolate: Fold in 3/4 cup (130 g) mini chocolate chips just before baking.
- Cupcake Format: Divide batter among 24 lined muffin cups and bake at 350°F (175°C) for 18–20 minutes.
Storage & Make-Ahead
Cover the cooled, glazed cake tightly and store at room temperature for up to 3 days; the mayonnaise keeps the crumb notably moist. For longer storage, refrigerate up to 5 days (bring to room temp before serving). To freeze, wrap individual squares in plastic, then foil, and freeze up to 2 months; thaw at room temperature still wrapped. The glaze can be made up to 2 days ahead—rewhisk and warm 5–10 seconds if thick.
Nutrition (per serving)
Approximate values per slice (1/16 of cake): 290 calories; 43 g carbohydrates; 11 g fat; 3 g protein; 1 g fiber; 220 mg sodium; 24 g sugars.
