Preheat oven to 350°F. Grease two 9 x 2-inch-round layer-cake pans. Line bottoms of pans with wax paper. Grease paper. Lightly dust inside of pans and the greased wax paper with flour.
Begin melting butter in a large, heavy saucepan over medium heat. When butter is half melted, remove the saucepan from the heat. Stir in the chocolate. Stir the mixture until the butter and chocolate are completely melted.
Stir in brown sugar, vanilla and salt. Stir in eggs.
Add baking soda in pinches, breaking up any lumps with fingers; stir thoroughly to combine. Stir in flour just until mixture is well blended. Stir in water.
Pour batter evenly into prepared pans, dividing equally.
Bake in preheated 350°F oven for 30 to 35 minutes or until cake springs back when lightly pressed with tip of finger and pulls away slightly from sides of pan.
Cool the cakes in pans on wire racks for 10 minutes. Invert cakes onto racks. Remove pans; remove the wax paper. Cool the cakes completely.
When the layers are completely cooled, fill and frost with the Chocolate Sour-Cream Icing. (If the icing begins to harden in the bowl while spreading, beat occasionally to soften.)
Gently melt semisweet chocolate pieces in the top of a double boiler set over hot, not boiling water. Or microwave the chocolate in a microwave-safe bowl at 100% power for 45 seconds; stir the chocolate until well blended and smooth. Whisk in sour cream until well blended and smooth.