Preheat oven to 325 F. Grease and flour a 9x5-inch loaf pan. Set aside.
Add chopped chocolate to a heat-proof bowl.
2 ounces semisweet baking chocolate
Pour boiling water over chocolate, and stir until melted.
⅔ cup boiling water
Whisk in cocoa powder until combined. Set aside.
½ cup Dutch-processed cocoa powder
In a large bowl, beat butter and sugars together until well-combined, about 2-3 minutes on medium-high.
1 cup butter, 1 cup granulated sugar, ½ cup brown sugar
Add vanilla, and mix in eggs one at a time until combined, about 2 minutes on medium.
2 teaspoons vanilla extract, 5 large eggs
Stir in chocolate mixture.
Add flour, salt, and baking powder. Stir in just until combined. Do not overmix.
1 cup all-purpose flour, 1 teaspoon salt, ½ teaspoon baking powder
Pour batter to prepared pan.
Bake 50-60 minutes, or until a toothpick inserted in the center comes out with a few fudgy crumbs.
Let cool in pan 10 minutes.
Remove to wire rack, and cool to room temperature before slicing.
Notes
Flour: To properly measure the flour, either weigh it or aerate (sift or stir), spoon, and level.
Checking for doneness: The toothpick should have a just a few fudgy crumbs - similar to what you look for when baking brownies. Wet batter means that it needs to bake longer.