Heat the oven to 350°F with a rack in the middle position. Mist a 9-by-5-inch loaf pan with cooking spray, then line it with an 8-by-14-inch piece of kitchen parchment, allowing the excess to overhang the long sides of the pan.

In a medium saucepan over medium, bring 1 inch of water to a simmer. Put the chocolate and the butter in a large heatproof bowl and set the bowl on top of the saucepan; be sure the bottom does not touch the water. Stir occasionally until the chocolate and butter are melted. Remove the bowl from the pan and cool until barely warm to the touch, stirring occasionally. Meanwhile, in a medium bowl, whisk together the flour, sugar, cocoa, baking powder and salt.

Add the eggs to the cooled chocolate mixture; whisk until well combined. Add the dry ingredients and stir with a silicone spatula until evenly moistened; the mixture will be dryish and thick. Gradually stir in about half of the boiling water; when fully incorporated, whisk in the remaining boiling water. The batter will be smooth, glossy and fluid. Pour into the prepared pan. Bake until the center of the cake rises, forming deep fissures on the surface, and a toothpick inserted at the center comes out with a few crumbs attached, 55 to 60 minutes.

Cool in the pan on a wire rack for about 20 minutes. Lift the cake out of the pan using the parchment and set it directly on the rack. Cool to room temperature. Peel off and discard the parchment before slicing.