Set a rack at the middle level of the oven and preheat to 350 degrees.
Sift flour, baking soda and salt, once then set aside. With an electric mixer set at medium speed, beat butter until soft and light. Add sugar and continue beating until very light, about 5 minutes. Beat in chocolate, then eggs, 1 at a time. Continue beating until light and smooth.
Beat in half the sour cream, then half the flour mixture, scraping bowl and beater(s). Repeat with remaining sour cream and flour mixture, scraping again. Combine vanilla and boiling water and gently beat into batter. Pour batter into prepared pan and bake for about 45 minutes, until firm and well risen. Cool in pan on rack for 5 minutes, then unmold and cool on a rack. For the frosting, combine all ingredients in bowl of mixer. Whisk to combine then place over a pan of simmering water, gently whisking until mixture is hot and sugar is dissolved. Use electric mixer on medium speed to beat until cooled, but not dry. To finish, cover top and side of cake with frosting, swirling it from the center outward.