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Special equipment: a 9-by-5-inch loaf pan
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Position a rack in the lower third of the oven, and preheat to 325 degrees F. Butter a 9-by-5-inch loaf pan.
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Sift the flour and salt into a medium bowl.
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Whisk together the eggs, sour cream and vanilla in a small bowl.
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Beat the butter in a large bowl with an electric mixer on medium-high speed until smooth.
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Gradually add the sugar, 1/4 cup at a time, to cream and lighten the butter.
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Continue to beat the butter mixture until light and fluffy, about 5 minutes.
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Scrape down the bowl as needed.
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Reduce the mixer speed to low.
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Beat in the flour, 1/4 cup at a time, then beat the batter for 30 seconds.
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Gradually beat in the egg mixture.
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(Take care not to overmix.)
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Transfer about 1/3 of the batter to a medium bowl, and stir in the hot chocolate mix.
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Spoon 1/2 the remaining vanilla batter into the prepared loaf pan, and smooth over the top with a spatula.
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Using 1/2 the hot chocolate batter, drop spoonfuls on top of the vanilla batter.
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Repeat with the remaining vanilla and hot chocolate batters.
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Run a skewer through the loaf pan to help swirl the 2 batters together.
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Tap the pan on the counter.
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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.
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Let the cake cool in the pan for a few minutes, then invert and let cool completely on a rack.
