• 2 oz. bittersweet chocolate, chopped
• 1/2 c. boiling water
• 3/4 c. Dutch-processed cocoa powder, plus extra for pan
• 2 c. flour
• 2 c. walnuts or pecans, finely chopped
• 2 c. powdered sugar
• 1 tsp. salt
• 5 eggs, at room temperature
• 1 tbsp. vanilla extract
• 1 c. granulated sugar
• 3/4 c. packed light brown sugar
• 11/4 c. (2 1/2 sticks) unsalted butter, at room temperature, plus extra for pan
• 4 oz. bittersweet chocolate, finely chopped
• 1/2 c. heavy cream
• 2 tbsp. granulated sugar
• 2 tbsp. water
To prepare cake: Adjust oven rack to lower-middle position and preheat oven to 350 degrees. Generously butter a 12-cup Bundt pan and dust with cocoa powder, tapping out excess. Place bittersweet chocolate in a small heatproof bowl. Add boiling water, whisk until smooth, cool to room temperature and reserve.
In a large bowl, whisk together cocoa, flour, walnuts (or pecans), powdered sugar and salt and reserve.
In a large measuring cup, whisk together eggs and vanilla extract and reserve.
In a large bowl, using an electric mixer on medium-high speed, beat granulated sugar, brown sugar and butter until light and fluffy, about 2 minutes. Reduce speed to low, add egg mixture and mix until just combined, about 30 seconds. Add chocolate mixture and mix until just combined, about 30 seconds. Add flour mixture and mix until just combined.
Scrape batter into prepared pan, smooth batter with a rubber spatula and bake until edges are beginning to pull away from pan, about 45 minutes; do not overbake (do not use a cake tester, toothpick or skewer to test the cake).
Remove to a wire rack and cool, upright, for 1 1/2 hours. Invert cake onto serving plate and cool completely, at least 2 hours.
To prepare glaze: Place chocolate in a heatproof bowl. In a small saucepan over high heat, combine cream, granulated sugar and water and bring to a boil. Pour liquid over chocolate and let sit for 30 seconds. Using a whisk or a rubber spatula, gently stir chocolate and cream together in small circles, starting at center of bowl and working your way out in increasingly larger concentric circles.
Leave glaze at room temperature until it thickens just enough to pour in a ribbon, about 20 minutes. Drizzle glaze over cake and let sit for at least 10 minutes before serving. Cover tightly and store at room temperature for up to 2 days.