Special equipment: a 9-by-5-inch loaf pan.
Position a rack in the lower third of the oven, and preheat to 160°C. Butter a 9-by-5-inch loaf pan.
Sift the flour and salt into a medium bowl. Whisk together the eggs, sour cream and vanilla in a small bowl.
Beat the butter in a large bowl with an electric mixer on medium-high speed until smooth. Gradually add the sugar, 1/4 cup at a time, to cream and lighten the butter.
Continue to beat the butter mixture until light and fluffy, about 5 minutes. Scrape down the bowl as needed.
Reduce the mixer speed to low. Beat in the flour, 1/4 cup at a time, then beat the batter for 30 seconds. Gradually beat in the egg mixture. (Take care not to overmix.)
Transfer about 1/3 of the batter to a medium bowl, and stir in the hot chocolate mix. Spoon 1/2 the remaining vanilla batter into the prepared loaf pan, and smooth over the top with a spatula. Using 1/2 the hot chocolate batter, drop spoonfuls on top of the vanilla batter. Repeat with the remaining vanilla and hot chocolate batters. Run a skewer through the loaf pan to help swirl the 2 batters together. Tap the pan on the counter.
Bake until a toothpick inserted in the center comes out clean and the top springs back when lightly pressed, 1 hour to 1 hour and 30 minutes.
Let the cake cool in the pan for a few minutes, then invert and let cool completely on a rack.
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For best results, don't use hot chocolate or cocoa mixes that contain powdered milk or other dairy products, which can change the structure and texture of the cake.
From Food Network Kitchen