Preheat the oven to 350 degrees F (180 degrees C) and grease a 9x13-inch baking dish.

In a large mixing bowl, combine flour, sugar, cocoa, baking powder, baking soda, and salt. Stir to mix thoroughly.

Combine boiling water and butter in a bowl, and stir until the butter is melted. Allow about 5 minutes for the mixture to cool. While stirring briskly, slowly add eggs. When the egg and butter mixture is well blended, stir in vanilla.

Add the wet mixture to the dry mixture and beat about 2 minutes. Stir in nuts, if using.

Pour batter into the prepared pan, and bake in the center of the preheated oven until a wooden pick inserted in the middle of the cake comes out clean, 30 to 35 minutes.

Cool completely on a wire rack, about 15 minutes. When cool, place powdered sugar in a mesh sieve, or a sifter, and dust the cake with powdered sugar, if desired. Cut into squares and serve.