To prep the pan:
1 tbsp. Dutch-processed cocoa powder
1 tbsp. unsalted butter, melted
For the cake:
½ cup boiling water
2 oz. bittersweet chocolate, chopped
2 cups (10 oz.) all-purpose flour
¾ cup (2¼ oz.) Dutch-processed cocoa powder
2 cups (8 oz.) confectioners’ sugar
1 tsp. salt
5 large eggs, at room temperature
1 tbsp. vanilla extract
1 cup (7 oz.) granulated sugar
¾ cup packed (5¼ oz.) light brown sugar
20 tbsp. (2½ sticks) unsalted butter, at room temperature
For the glaze:
¾ cup heavy cream
¼ cup light corn syrup
8 oz. bittersweet chocolate, finely chopped
½ tsp. vanilla extract
Preheat the oven to 350˚ F. In a small bowl, whisk together the cocoa powder and melted butter. Use a pastry brush to evenly coat the inside of a 12-cup bundt pan.
In a medium bowl, pour the boiling water over the chopped chocolate. Let stand for a minute, then whisk until smooth. In a medium bowl, combine the flour, cocoa powder, confectioners’ sugar, and salt. Combine the eggs and vanilla in a liquid measuring cup and beat lightly.
In the bowl of an electric mixer, combine the granulated sugar, brown sugar, and butter. Beat on medium-high speed until light and fluffy, about 2 minutes. With the mixer on low speed, add in the egg mixture just until combined. Add the chocolate mixture and mix until incorporated. Blend in the dry ingredients until just combined. Spread the batter into the prepared pan. Bake for about 45 minutes or until the edges of the cake begin to pull away from the pan.
Allow the cake to cool on a wire rack for 1½ hours before inverting onto a cake platter. Allow to cool completely, about 2 more hours.
To make the glaze, combine the cream, corn syrup, and chocolate in a small saucepan. Heat over medium heat, stirring frequently, until smooth. Blend in the vanilla. Set aside for about 30 minutes, until slightly thickened. Drizzle the glaze over the cake and let set for at least 10 minutes before slicing.