Prepare the Oven and Pan: Heat the oven to 325 F (165 C) and grease and flour a 9-by-5-bt-3-inch loaf pan.

Melt the Chocolate: Bring the water to a boil in a saucepan. Put the chocolate in a small bowl and pour the boiling water over it.

Combine Dry Ingredients: Combine the flour, baking soda, baking powder, and salt in a small bowl; whisk or stir to blend thoroughly.

Cream the Butter and Sugar: In a mixing bowl with electric mixer, beat the butter and brown sugar until light, about 3 minutes.

Make the Batter: Add the eggs to the butter and sugar mixture, one at a  time, beating well after each addition. Beat in the vanilla. Add one-third of the dry ingredients and mix. Repeat with another 1/3 of the dry ingredients and the remaining sour cream. mix in the remaining flour until blended. With the mixer on low, incorporate the chocolate mixture, beating just until blended.

Bake and Cool: Spread the batter in the prepared loaf pan and bake for 55 to 65 minutes or until a toothpick or wooden skewer comes out with just a few crumbs clinging.  Cool in the pan on a rack for 15 minutes, then remove the cake to the rack to cool completely.

Decorate the Cake: Dust with powdered sugar, frost, or glaze.