Prepare an 8 1/2 x 4 1/2-inch metal loaf pan with flour-based baking spray and a parchment sling, then heat the oven to 350°F with the rack in the middle. Leave parchment extending above the long sides so you can lift the loaf without tipping it out.

Melt the chocolate in a medium microwave-safe bowl using 1-minute intervals, then melt the butter separately using 30-second intervals and let both cool slightly. The chocolate should look smooth and glossy, and the butter should be fully liquid without remaining solid pieces.

Combine the flour, Dutch-processed cocoa, baking soda, and salt in a medium bowl with a whisk. A uniform color with no visible cocoa pockets helps distribute the dry ingredients evenly.

Whip the eggs and sugar in a large bowl with a hand mixer or stand mixer at medium-high speed for about 4 to 4 1/2 minutes. Look for a pale, fluffy mixture that has nearly doubled in volume and falls from the beaters in a thick ribbon.

Blend the melted chocolate, milk, and vanilla into the whipped egg mixture, stopping when they are mostly incorporated. The mixture should turn chocolate brown, though a little unevenness is fine before folding.

Use a spatula to fold the dry mixture into the wet mixture, then fold in the melted butter. Sweep down through the batter and lift it over itself until no dry pockets or butter streaks remain, avoiding vigorous beating.

Transfer the batter to the prepared pan, add a few semi-sweet chocolate chips on top if desired, and bake at 350°F for 55 to 60 minutes. A toothpick inserted into the center should emerge carrying damp crumbs without a coating of liquid batter.

Rest the baked loaf in its pan for about 15 to 20 minutes, then lift it onto a wire rack to finish cooling. The loaf should feel more settled before lifting, while the rack lets moisture escape from the bottom and sides.