Preheat oven to 350 degrees Fahrenheit. Grease a 9x9" square pan with cooking spray.
Whisk together sugar, water, mayonnaise, and vanilla. Set aside. 1 cup granulated sugar, 1 cup water, 1 cup mayonnaise, 1 teaspoon vanilla extract
In a separate bowl stir flour, baking soda, cocoa powder, and salt together. 2 ¼ cups all-purpose flour, 1 ½ teaspoons baking soda, ¼ cup unsweetened cocoa powder, ¼ teaspoon salt
Add dry ingredients to wet ingredients and stir until just combined. Do not overmix.
Pour batter into the prepared pan. Bake for 30-35 minutes or until a toothpick comes out clean. Allow cake to cool completely before frosting.
Make the frosting:
Make frosting by beating together butter, salt, lemon juice and vanilla extract until light and fluffy, about 1 minute. ½ cup unsalted butter, ½ teaspoon salt, 1 teaspoon lemon juice, 1 teaspoon vanilla extract
Mix in the cocoa powder, 1 cup powdered sugar and heavy cream. Mix to combine. ¼ cup unsweetened cocoa powder, 3 cups powdered sugar, ¼ cup heavy cream
Slowly add in the rest of the powdered sugar ½ cup at a time until all 3 cups have been incorporated.
Turn your stand mixer on high and beat for the next 5 minutes until light and fluffy. Frost cake and cut into squares.
Notes
Store in an airtight container at room temperature.The calories shown are based on the cake being cut into 9 pieces, with 1 serving being 1 slice of frosted cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**