Step 1 Heat the oven to 350 degrees. Butter a 10-inch springform pan with a removable bottom, line it with parchment paper, and set aside.
Step 2 Sift together the flour, cocoa, salt and baking soda. In a separate bowl mix together the water, oil and vanilla.
Step 3Beat the eggs in the bowl of an electric mixer, then mix in the sugar. Mix in the water mixture in thirds, alternating with thirds of the sifted dry ingredients, scraping down the sides of the bowl with a rubber spatula between each addition. Mix until smooth.
Step 4Pour the batter into the pan and bake for 20 minutes; reduce the oven temperature to 325 degrees and bake until a toothpick inserted into the middle of the cake comes out clean, 65 to 75 minutes.
Step 5Place the pan on a rack and cool the cake to room temperature. Remove it from the pan and split into 3 equal parts horizontally. Trim the top layer of the cake to make it flat. Place 1 round of cake back into the pan, then prepare the mousse.