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1
Get ready: center oven rack; preheat oven to 350u00b0; butter a 9 1/2 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.
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2
Make the cake: sift together the flour, cocoa, baking powder, baking soda, and salt.
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3
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.
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4
Add in the eggs one at a time, beating for 1 minute after each one goes inches.
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5
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.
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6
Give the batter a last stir with a sturdy rubber spatula and scrape the batter into the pan.
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7
Bake for 60-70 minutes, or until a knife inserted into the cake comes out clean.
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8
If, after about 45 minutes, the cake looks like it is browning too quickly, cover it loosely with a foil tent.
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9
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.
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10
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.
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11
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.
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12
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.
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13
Cover with the middle layer and spread that layer with the remaining jam.
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14
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.
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15
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.
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16
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.
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17
Remove from heat and cover the sides and top of the cake with the still-warm frosting.