Preheat oven to 350F (175C). Grease and flour a 9-inch round cake pan.
In a large bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.
In a separate bowl, combine sugar, buttermilk, vegetable oil, egg substitute, and vanilla extract. Mix well.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix.
Slowly pour in the boiling water and stir until the batter is smooth and thin.
Pour the batter into the prepared cake pan.
Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs attached.
Let the cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.
Frost with your favorite chocolate frosting.
For a richer chocolate flavor, add cup of chocolate chips to the batter. Adjust baking time if using a different size or shape of pan. To prevent sticking, line the bottom of the cake pan with parchment paper.