Instructions

• Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan.
• In a large bowl, whisk together 2 cups all-purpose flour, 1 cup granulated sugar, 1/2 cup unsweetened cocoa powder, and 2 teaspoons baking soda until evenly combined.
• Add 1 cup mayonnaise, 1 cup lukewarm water, and 1 teaspoon vanilla extract to the dry ingredients. Stir until the batter is smooth and uniform, scraping the bowl sides as needed.
• Pour the batter into the prepared 9x13-inch pan and smooth the top with a spatula.
• Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean. Remove from oven and place the pan on a wire rack to cool.
• While the cake cools, make the frosting: in a medium saucepan combine 1/3 cup milk, 1/3 cup butter, and 1 1/4 cups granulated sugar. Cook over medium heat, stirring, until the mixture comes to a boil.
• Boil the mixture for 30 seconds while stirring, then remove the saucepan from heat.
• Stir in 1 cup semi-sweet chocolate chips and 1 teaspoon vanilla extract until the chocolate is fully melted and the frosting is smooth.
• Once the cake has cooled (it should be cool to the touch), pour the warm frosting evenly over the cake and spread if necessary. Allow the frosting to set before slicing and serving.