Tips for success:The first time you make any recipe, read through it a few times before starting, and make it exactly as written -- baking is science.Make sure all ingredients are at room temperature.As with all baking, using a scale is a game changer. It is how all professional pastry chefs bake.
Preheat oven to 375 degrees. Grease and flour two 9-inch cake pans, or line cupcake tins with papers if making cupcakes.
In a large bowl or bowl of an electric mixer, whisk the flour, sugar, cocoa powder, baking powder, baking soda and salt together, to evenly mix and break up any clumps.
In a separate bowl or measuring cup, whisk the eggs, milk, oil and vanilla together. (DON'T ADD the boiling water yet.)
Add the wet ingredients to the dry ingredients, and whisk or mix on medium speed for a few minutes, scraping down your bowl a few times during your mixing.
Carefully pour in your boiling water or coffee, and mix well with a whisk. Make sure to incorporate any batter on the bottom and sides of the bowl. The batter will be thin -- don't worry it is supposed to be.
Rest your batter for at least an hour and up to 24 hours.
Pour your batter into the prepared pans. If making cupcakes, fill each cup ⅔ full.
Bake on the center oven rack for 10 minutes at 375 degrees, then reduce the oven temperature to 350 degrees and continue baking for an additional 20-25 minutes for cake (or 10-15 minutes for cupcakes). A toothpick inserted in the center should come out clean.
Cool for 10 minutes in pans, and then remove from pans and place on wire racks to cool. I like to put a piece of parchment under my cakes when cooling, so the rack doesn't dig into the fragile cake and leave marks.
Once completely cooled, frost with your favorite frosting, lightly sweetened whipped cream or dust with confectioners' sugar.
Notes
* Using coffee instead of water gives a deeper, darker chocolate flavor.