Get ready: center oven rack; preheat oven to 350°; butter a 9 ½ x 5 inch loaf pan, dust the inside with flour and top out the excess; place the pan on a insulated baking sheet or on two regular baking sheets stacked on top of one another.
Make the cake: sift together the flour, cocoa, baking powder, baking soda, and salt.
Using a stand mixer fitted with a paddle attachement, beat the butter and sugar together at medium speed for about 3 minutes, or until very light and fluffy.
Add in the eggs one at a time, beating for 1 minute after each one goes inches.
Decrease the speed to low and mix in the sour cream; still working on low speed, add the dry ingredients and mix only until they disappear into the batter.
Give the batter a last stir with a sturdy rubber spatula and scrape the batter into the pan.
Bake for 60-70 minutes, or until a knife inserted into the cake comes out clean.
If, after about 45 minutes, the cake looks like it is browning too quickly, cover it loosely with a foil tent.
Transfer the pan to a cooling rack and let the cake rest for about 5 minutes before turning it out onto the rack; cool to room temperature upside down.
Fill the cake: bring the jam and water to a boil over low heat or in a microwave oven; stir to smooth it, and keep it at hand.
The top of the cake will become the bottom layer, so if it is uneven and your cake rocks a bit on the counter, turn it right side up and use a long serrated knife to level it.
Using a serrated knife, cut the cake into 3 even layers; put the first layer (originally the top of the cake) cut side down on a long serving plate and spread half of the jam over it.
Cover with the middle layer and spread that layer with the remaining jam.
Lift the top layer into place, cut side down; use a small pastry brush to chase away any crumbs on the top or sides of the cake.
Make the frosting: fit a heatproof bowl into a pan of gently simmering water, add in the chocolate and warm, stirring occasionally, until the chocolate is melted.
Still working over the hot water, stir in the sour cream; don’t be concerned if the cream tightens—just keep stirring gently and the frosting will become smooth and glossy.
Remove from heat and cover the sides and top of the cake with the still-warm frosting.