Preheat the oven to 350°F. Spray a bundt pan with nonstick cooking spray.

Combine the cocoa, chocolate, and espresso in a large bowl. Pour the boiling water into the bowl and cover. Let stand for 5 minutes. In a small bowl whisk together the flour, baking soda, and salt. Whisk the chocolate mixture until it is smooth. Let cool to room temperature. When it has cooled, whisk in the sour cream.

In the bowl of an electric mixer, beat the butter, brown sugar, and vanilla for about 4 minutes, or until pale and fluffy. Add the eggs, one at a time, until combined. On low speed add the flour mixture and the melted chocolate mixture alternatively, beginning and ending with the flour, until the batter is smooth.

Pour the batter into the prepared pan. Bake the cake for about 50 minutes, or until a toothpick comes out with moist crumbs. Let cool on a rack for 10 minutes, then turn the cake out and let cool completely.

In a small saucepan heat the cream and corn syrup over medium heat until hot. Remove from heat and add the chocolate, whisking until smooth. Let cool slightly until thickened. Drizzle over the cake. Let the glaze set at room temperature, about 10 minutes, before serving.