Instructions: Preheat oven to 350 degrees. Butter a 10-inch springform pan and line the bottom with a disk of parchment or wax paper.

Mix flour, baking soda and salt, sift once and 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, one at a time, continuing to beat 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 and place over a pan of simmering water, gently whisking until mixture is hot and sugar is dissolved. Whip on medium speed until cooled, but not dry.

To finish, cover top of cake with frosting, swirling it from the center outward. To serve, cut the cake with a moist knife, wiping the blade with a wet cloth between each slice.

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