Chocolate Mayonnaise Sheet Cake 

Ingredients

For the cake

Butter, for greasing the pan

2 cups (242g) all-purpose flour

1 cup (120g) unsweetened Dutch processed cocoa powder 

1 1/2 teaspoons baking powder

1 1/2 teaspoons baking soda

1/4 teaspoon salt

3/4 cup (177ml) hot water

2 tablespoons (30g) instant coffee powder or instant espresso powder

2 large eggs

1 cup (220g) sugar

1/2 cup (100g) brown sugar, lightly packed

1 cup (260g) mayonnaise

1 tablespoon vanilla extract

For the frosting

1/2 cup (80g) finely chopped semisweet or bittersweet chocolate 

1/2 cup (113g) unsalted butter 

2 tablespoons (30g) unsweetened Dutch processed cocoa powder 

1 cup (130g) powdered sugar

1/2 cup heavy cream

Special Equipment

9x13-inch baking pan

Method:

To make the cake

Preheat the oven:

Preheat the oven to 350°F (180°C). Brush a 9x13-inch sheet pan with a little butter and set aside.

Mix the dry ingredients:

In a medium bowl, stir together the all-purpose flour, cocoa powder, baking powder, baking soda and salt until evenly combined. Set aside.

Mix wet ingredients:

In a glass measuring cup, add the instant coffee to the hot water. Mix until dissolved and set aside.

Add the eggs:

Add the eggs one at a time and beat well. Make sure to scrape down the sides of the bowl with a spatula periodically to ensure everything is getting incorporated. Pour in the vanilla and beat to mix.

Beat the mayonnaise and sugars:

In a large mixing bowl, beat the mayonnaise and white and brown sugars using an electric beater just until pale and creamy.

Fold the wet and dry ingredients together:

Roughly divide the dry ingredients in thirds and add the first third to the egg mayonnaise mixture with half of the hot coffee. Beat until combined. Add the second part of the dry ingredients with the other half of the hot coffee and beat well. 

Add the final third of the dry ingredients and give it a good mix. The batter should be neither stiff nor runny.

Bake the cake:

Pour the batter into the prepared tin and bake at 350°F (180°C) for 28-30 minutes or until a toothpick inserted into the center comes out with moist crumbs (not wet cake batter but sticky chocolate crumbs). Let the cake cool completely before frosting.

To make the frosting

Melt the chocolate:

In a small microwaveable bowl, melt the dark chocolate in a microwave using 20-second intervals until smooth, stirring between each interval. (It should take 40-70 seconds total). Set the chocolate aside to cool.

Beat the butter and sugar:

In a large bowl, use an electric mixer to beat the butter until it’s creamy and fluffy.

Add the powdered sugar to the creamed butter. Starting on the lowest speed, beat the mixture together. As the sugar gets incorporated, increase the speed and beat the frosting for 2-3 minutes on high speed.

Add the cocoa powder:

Add the cocoa powder to the frosting and beat well until combined. Make sure to scrape down the sides every now and then for a smooth frosting.

Add the cream and cooled melted chocolate:

Add the cream and melted dark chocolate to the frosting and beat well until combined. Let this set in the fridge for 10 minutes before frosting the cake.

Assemble the cake

Frost:

Spread the frosting over the cooled cake and create a swirls using your spatula or the back of a spoon.

Serve and store:

Slice and serve the cake. 

Once the cake is baked, leave it in the baking tin and cover it with plastic wrap. Leave it at room temperature for 1 day or refrigerate for up to 7 days. 

Once the sheet cake is frosted you can store it in the fridge for 3 days, however when you are ready to eat, let the cake come to room temperature before slicing as the frosting will be hard.

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