Whisk flour sugar cocoa baking soda baking powder and salt until evenly combined.
Mix and Bake
Whisk water mayonnaise and vanilla until smooth
Pour wet ingredients into dry ingredients and stir just until no dry flour remains
Divide batter evenly among liners filling about 2 3 full
Bake until a toothpick inserted in the center comes out with a few moist crumbs 18 to 22 minutes
Cool in pan 5 minutes then move to a rack to cool completely.
Optional Frosting
Beat butter until smooth
Add powdered sugar cocoa salt vanilla and milk and beat until fluffy adjusting milk for spreadable consistency
Frost cooled cupcakes.
Notes
1. Use full fat mayonnaise for best texture 2. Do not overmix after combining wet and dry 3. Replace water with cooled coffee for deeper chocolate flavor 4. Cupcakes keep airtight 3 days at room temperature or freeze unfrosted up to 2 months