Step 1 Adjust a rack to the center of the oven and heat to 350 degrees. Butter or spray the pans and line them with parchment or wax paper cut to fit. Butter or spray the paper.
Step 2 Place the chocolate and butter in the top of a double boiler and melt, stirring until smooth.
Step 3Stir the sour cream and baking soda together in a small bowl and set aside.
Step 4When the chocolate and butter are melted, transfer to the bowl of an electric mixer. Add sugar, vanilla and salt and beat just to mix. Add the eggs one at a time, beating until mixed after each addition. Mix in the sour cream mixture. On low speed, add the flour, scraping the bowl with a spatula and beating only until smooth. Then, on the lowest speed, very gradually add the boiling water, scraping the bowl and beating only until smooth.
Step 5Pour the mixture (it will be thin) into the two pans.
Step 6Bake for 25 to 28 minutes, until the tops spring back lightly when gently pressed with a fingertip.
Step 7Cool the cakes in their pans for 10 minutes. Then cut around the layers with a small sharp knife and release. Cover with a rack, invert, remove pan, cover with another rack and invert again to cool right-side up. Cool for at least 2 hours before assembly. For a two-layer cake, level the layers and frost. For a four-layer cake, level the layers, cut each in half horizontally and frost.