Preheat oven to 350 degrees. Butter a 12-by-17-by-1-inch rimmed baking sheet and line with parchment, leaving an overhang folded over edges. Butter parchment. Melt butter and uncut chocolate in a large bowl set over a pot of simmering water. Stir until smooth. Remove from heat.
Whisk in sugar, then eggs, then cocoa powder and salt. Fold in flour until just combined. Fold in chocolate chunks.
Pour batter into pan. Bake, rotating halfway through, until a toothpick inserted into center comes out with very moist crumbs, 45 to 48 minutes. Let cool completely, about 2 hours. Remove from pan and cut into 2-inch squares.