favorite loaf pan (Williams-Sonoma Goldtouch Nonstick) is slightly smaller than a standard 9 by 5-inch pan. If you’re using a standard pan, begin to check the cake for doneness after 55 minutes. Cooking time includes cooling time.
1. Adjust oven rack to lower-middle position and heat oven to 325 degrees. Grease and flour 8½ by 4½-inch loaf pan. Combine flour and salt in bowl. Pour water over cocoa and chocolate in second bowl and stir until chocolate is melted and no dry streaks of cocoa remain. Let mixture cool 5 minutes.
2. Using stand mixer fitted with paddle, beat butter, cocoa-chocolate mixture, granulated sugar, brown sugar, and vanilla on medium-high speed until fluffy, 2 to 3 minutes. Add eggs, one at a time, and beat until combined. Reduce speed to low and add flour mixture in 3 additions, scraping down bowl as needed, until just combined. Give batter final stir by hand (it may look curdled).
3. Scrape batter into prepared pan and gently tap pan on counter to release air bubbles. Bake until toothpick inserted in center comes out clean, 60 to 70 minutes. Cool cake in pan on wire rack for 10 minutes. Remove cake from pan and cool for 2 hours. Serve.