In a large bowl, combine the sugar, flour, cocoa, baking powder, baking soda, and salt. Add the eggs, milk, oil, and vanilla. Beat for a couple minutes until well combined. Stir in the orange zest. Add the boiling water, orange juice and stir. The batter will be thinner than you might expect. Pour the batter into the prepared pans. Bake for about 35 minutes, until a toothpick inserted comes out clean. Cool completely before filling and frosting.