1. Have all the ingredients (except the warm water) at room temperature.
2. Position a rack in the lower third of an oven and preheat to 325 degrees F. Grease and flour a classic bundt cake pan; tap out excess flour.
1. In a bowl, whisk together the cocoa powder and warm water until blended; let cool to room temperature.
2. Over a sheet of waxed paper, sift together the flour, baking soda, baking powder and salt; set aside.
3. In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until creamy and smooth, about 30 seconds. Add the sugar and continue beating until light and fluffy, about 5 minutes, stopping the mixer occasionally to scape down the sides of the bowl. Add the eggs one at a time, beating well after each addition.
4. Add the vanilla and sour cream to the cooled cocoa mixture and stir to combine. Reduce the speed to low and add the flour mixture in three additions, alternation with the cocoa-sour cream mixture and beginning and ending with the flour. Blend each addition until just incorporated, stopping the mixer occasionally to scrape down the sides of the bowl.
5. Spoon the batter into the prepared pan, spreading the batter so the sides are higher that the center.
6. Bake the cake at 325 degrees until the cake begins to pull away from the sides of the pan and a toothpick inserted into the center of the cake comes out clean, about 50 minutes.
7. Transfer the pan to a wire rack and let the cake cool upright in a pan for 15 minutes. Set the rack over a sheet of waxed paper, invert the pan onto the rack and lift off the pan. Let the cake cool completely, about 1 hour, before glazing.
1. In a small saucepan over low heat, combine the chocolate and butter and heat, stirring occasionally, until the chocolate and butter melt and the glaze is smooth and shiny, 2 to 3 minutes.
2. Remove the pan from the heat, add the corn syrup and stir to combine.
3. Pour the glaze over the cake and let stand until the glaze is set 30 to 40 minutes, before serving.
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