1 cup all purpose flour
1 teaspoon salt
1/3 cup boiling water
3/4 cups Dutch-processed cocoa
2 ounces milk chocolate, chopped fine
16 tablespoons unsalted butter, softened
1 cup granulated sugar
1/4 cup packed light brown sugar
2 teaspoons vanilla
5 large eggs, room temperature
Adjust oven rack to lower-middle position and heat over to 325 degrees. Grease and flour a 8 1/2 x 4 1/2 loaf pan.
Combine flour and salt in bowl. Pour water over cocoa and chocolate in a second bowl and stir until chocolate is melted and no dry streaks of cocoa remain. Let mixture cool for 5 minutes.
Using a standard mixer fitted with paddles, beat butter, cocoa mixture, granulated sugar, brown sugar, and vanilla together on medium-high sped until fluffy, 2-3 minutes. Add eggs, one at a time, beating after each addition until combined.
Reduce speed to low and add flour mixture in 3 additions, scraping down bowl as needed, mixing until just combined. Give batter a final stir by hand (it may looked curdled).
Scrap batter into prepared pan and gently tap pan on counter to release any air bubbles. Bake until toothpick inserted in center comes out clean - 1hour and 10 minutes. Let cake cool in pan on wire rack for 10 minutes.
Remove cake from pan and let cool for 2 hours before cutting.