Preheat oven to 325 F. Grease and flour a 9x5-inch loaf pan. Set aside.
Add chopped chocolate to a heat-proof bowl.
Pour boiling water over chocolate, and stir until melted.
Whisk in cocoa powder until combined. Set aside.
In a large bowl, beat butter and sugars together until well-combined, about 2-3 minutes on medium-high.
Add vanilla, and mix in eggs one at a time until combined, about 2 minutes on medium.
Stir in chocolate mixture.
Add flour, salt, and baking powder. Stir in just until combined. Do not overmix.
Pour batter to prepared pan.
Bake 50-70 minutes, or until a toothpick inserted in the center comes out with a few fudgy crumbs.²
Let cool in pan 10 minutes.
Remove to wire rack, and cool to room temperature before slicing.
Notes
To properly measure the flour, either weigh it or aerate (sift or stir), spoon, and level.
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.