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Preheat oven to 325°F. Spray a large bundt pan (at least 15 cups) generously with baking spray or butter well, then dust with 3 tablespoons cocoa powder, knocking out excess.
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Heat coffee, buttermilk, whiskey, butter, and cocoa powder in a medium heavy saucepan over moderate heat, whisking, until butter is melted. You can also just throw all those ingredients in the microwave and heat on high until butter is melted, 1-2 minutes. Remove from heat (or microwave), then add sugar and whisk until dissolved, about 1 minute. Transfer mixture to a large bowl and cool 5 minutes.
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While chocolate mixture cools, whisk together flour, baking soda, and salt in a bowl. Whisk together eggs and vanilla in a small bowl, then whisk egg mixture into cooled chocolate mixture until combined well. Add flour mixture and whisk until just combined (batter will be thin and bubbly).
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Pour batter into bundt pan and bake until a wooden pick or skewer inserted in center comes out clean, 40 to 50 minutes.
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Cool cake completely in pan on a rack, about 2 hours. Loosen cake from pan using tip of a dinner knife, then invert rack over pan and turn cake out onto rack. Drizzle with caramel glaze, if desired.
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For the glaze, combine sugar and ½ cup water in a medium-size sauce pan. Stir gently to combine and bring mixture to a boil over medium-high heat. Don't stir it any more at this point. You don't want to cause sugar crystals to form on the sides of the pan.
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Reduce heat to maintain a steady simmer. If you notice any sugar crystals on the sides of the pan, take a pastry brush that's been dipped in water and hold it on the side of the pan above the crystal/s, the water from the brush will wash the crystals back into the sugar mixture. The reason you don't want any sugar crystals to form on the sides of the pan is because they can cause a reaction that will make the whole mixture crystalize.
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Simmer (without stirring) until mixture reaches a deep amber color, about 6-8 minutes. At this point, remove pot from heat and, standing back a bit, add cream and butter to sugar mixture. It will bubble up furiously for just a minute. That's why it's important to stand back. Once the bubbling settles down, return pan to heat and bring the mixture back to a simmer.
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Simmer until thick and reduced to a molasses consistency. You might have to remove the pot from the heat and let the bubbling settle down to check the consistency. If it's still too thin, return to heat and continue to simmer. Mixture will thicken a bit on cooling, so don't let it get much thicker than molasses. This will take from 10-15 minutes.
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When glaze reaches the right consistency, remove from heat, add sea salt and vanilla and stir. Allow caramel mixture to cool to lukewarm before drizzling over cake.