Preheat oven to 375°F Line 12 regular-sized muffin tin with paper or foil liners.
Combine flour, cocoa, sugar, baking soda, and salt in a blender. Cover with the lid and blend on medium-speed until blended. Pour in the water, oil, egg and vanilla. Cover with the lid and blend on medium-high speed until sooth and well-blended, stopping to scrape down the sides once or twice.
Pour into prepared pan/cups, dividing evenly. Bake until toothpick inserted in center of one cupcake comes out clean, 15-17 minutes.
Transfer to rack to cool for about 10 minutes, and then remove cupcakes from the pan and set them on the rack to cool completely before frosting.