Put the chocolate in a heatproof bowl or the top of a double boiler and place it over, but not touching, a saucepan of barely simmering water (or the bottom of the double boiler). Stir until the chocolate is melted and smooth. Remove from the water and set aside to cool slightly.

Sift the flour, baking powder, baking soda, and salt into a medium bowl and set aside.

In a large bowl, using an electric mixer on medium speed, beat the butter and sugar until smoothly blended and creamy, about 2 minutes. Stop the mixer and scrape the sides of the bowl as needed during mixing. On low speed, mix in the melted chocolate. On medium speed, add the eggs one at a time, mixing until each is blended into the batter. Add the vanilla and beat until the mixture looks creamy and the color has lightened slightly, about 1 minute. Mix in the sour cream until no white streaks remain. On low speed, add half of the flour mixture, mixing just to incorporate it. Mix in the water. Mix in the remaining flour mixture until it is incorporated and the batter looks smooth. The batter is ready to bake, or for additions such as nuts, fruit, chocolate chips, or other flavorings.