Heat the oven to 350°F (175°C) and line two 12-cup muffin pans with paper liners. Position the oven rack in the center so the cupcakes bake evenly.

Whisk the flour, sugar, cocoa powder, baking soda, and salt together in a large bowl. Break up any cocoa lumps so the finished batter has an even color and texture.

In a separate bowl, whisk the full-fat mayonnaise, water, and vanilla extract until evenly blended. The mixture may look slightly creamy, which is exactly what you want.

Add the wet mixture to the dry ingredients and stir just until the batter is smooth. Do not overmix, because too much stirring can make the cupcakes less tender.

Divide the batter among the paper liners, filling each about two-thirds full. This leaves room for the cupcakes to rise without spilling over the edges.

Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Begin checking at 18 minutes so the cupcakes do not become dry.

Cool the cupcakes in the pans for about 5 minutes, then transfer them to a wire rack. Let them cool completely for 20–30 minutes before adding frosting.

For the frosting, beat the softened butter until smooth, then gradually mix in the powdered sugar and cocoa powder. Add the vanilla, salt, and enough milk or cream to create a spreadable consistency, then frost the cooled cupcakes.