Place a trivet at the bottom of a wide, deep pot. The pot will need to have a tight fitting lid. The pot should be wider than the cake pan, and deep enough for the cake to rise with the lid on. Coat the bottom of a 6-inch round cake pan with cooking spray and set aside.
In a small bowl, sift the flour, cocoa powder and baking soda, then mix in the salt.
Cover the pot and heat on high until the water boils. Reduce heat to low and steam, covered, for about 40-45 minutes, or until the cake batter is baked through and a skewer inserted in the center comes out clean.