Chocolate Soda Sheet Cake

This simple sheet cake pairs a boxed chocolate cake mix with carbonated water for an egg-free, oil-free batter, then adds a tangy chocolate-yogurt topping. Because cake-mix package weights and formulas vary, use a standard-size box and rely on the doneness cues as well as the baking-time range.

Equipment and timing

9-by-13-inch baking pan
Large and medium mixing bowls
Whisk and flexible spatula
Wire cooling rack

Yield: 12 pieces
Preparation: 15 minutes
Baking: 25 to 32 minutes
Cooling and chilling: 2 hours
Oven: 350°F/175°C

Ingredients

1 standard box (about 15.25 ounces) chocolate cake mix
1 can (12 fluid ounces) plain carbonated water or cola
Nonstick cooking spray
2 cups thick plain Greek yogurt or skyr
1/4 cup unsweetened cocoa powder
1/3 cup confectioners’ sugar, sifted
1 teaspoon vanilla extract
Pinch of fine salt
1 to 2 tablespoons milk, if needed
2 tablespoons finely chopped dark chocolate, optional

Method

Heat the oven to 350°F/175°C. Lightly coat the pan with cooking spray and line the bottom with parchment if easy removal is important.
Place the dry cake mix in the large bowl. Pour in the carbonated water and whisk just until no dry pockets remain, about 45 seconds. Do not follow the box directions or add the listed eggs and oil.
Spread the batter evenly in the pan. Bake for 25 to 32 minutes, rotating once, until the center springs back lightly and a toothpick comes out with a few moist crumbs but no wet batter.
Cool in the pan on a rack for 1 hour. Whisk yogurt, cocoa, confectioners’ sugar, vanilla, and salt. Add milk only if needed for a spreadable texture.
Spread the topping over the completely cool cake. Sprinkle with chopped chocolate if desired. Cover and chill for at least 1 hour before cutting into 12 pieces.

Success tips

The soda supplies bubbles but not the structure of eggs, so the crumb is softer than a conventionally prepared boxed cake. Avoid overmixing and cut only after chilling. If the cake sinks slightly, level the surface with the topping rather than baking it until dry. Use plain sparkling water for a neutral flavor or cola for a mild caramel note.

For a dairy-free topping, use a very thick cultured plant-based yogurt; thin yogurt may run. The cake itself may still contain milk or other allergens, so check the cake-mix label rather than assuming it is dairy-free.

Let the cake cool completely before spreading the topping. Even mild residual warmth can loosen yogurt and create a wet layer on the surface. For clean portions, chill first, use a long sharp knife, and wipe the blade between cuts.

Storage

Because of the yogurt topping, refrigerate the cake at 40°F/4°C or colder and use within 4 days. Keep it covered to prevent drying. Individual unfrosted pieces can be frozen for up to 2 months; thaw in the refrigerator and add topping after thawing.

Chocolate Soda Sheet Cake