• Preheat the oven to 325 °F. Grease a 9x5 inch metal loaf pan and line with a rectangle of parchment paper for easy removal.
• In a microwave-safe 2‑cup measuring cup or saucepan, bring ½ cup of the water to a boil. Stir in the chopped chocolate and let stand until the chocolate softens. Stir until melted and smooth. Add another ½ cup of water or room temperature coffee to help it cool quicker (total liquid 1 cup)
• Sift the flour, then whisk together with baking soda and salt. Set aside.
• Using a stand mixer with the paddle attachment, beat the butter until creamy. Add the brown sugar and continue beating for about 3 minutes on high, scraping down the bowl as needed.
• Beat in the eggs one at a time, mixing for about 30 seconds after each. Then beat in the vanilla extract.
• Add the sour cream to the chocolate mixture and just give it a little stir. No need to completely blend it, as it will all come together.
• Add the chocolate mixture and dry flour mixture alternately in 3 parts, stirring until everything is blended, then beat with the stand mixer on medium for about 1 minute. Do not overbeat, the goal is just to blend it evenly.
• Pour the batter into the prepared pan. Bake on the center rack at 325 °F for one hour, or until a toothpick inserted in the middle comes out clean. You may need up to an hour and 15 minutes depending on your oven and the shape of the loaf pan.
• Let the cake cool in the pan for about 15 minutes. Then invert the loaf onto a wire rack to cool completely.

Frosting

• Beat together softened butter and half of the powdered sugar until smooth. Beat in the cocoa powder and vanilla. Add milk and remaining powdered sugar, beating and scraping side of bowl until smooth and fluffy.