Preheat oven to 350 degrees Fahrenheit. Grease a 9x9" square pan with cooking spray.
Whisk together sugar, water, mayonnaise, and vanilla. Set aside.
In a separate bowl stir flour, baking soda, cocoa powder, and salt together.
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, lemon juice, and salt until light and fluffy, about 1 minute.
Mix in ¼ cup heavy cream, cocoa powder and 1 cup powdered sugar and combine.
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.
Store in an airtight container at room temperature.
Notes
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.**