Step 1Heat the oven to 350 degrees. Line a 9-inch square pan with parchment paper and spray with nonstick spray.
Step 2Place the butter and unsweetened chocolate in a double boiler over simmering water. Melt until smooth. Cool until warm to the touch, then stir in the sugar.
Step 3In a separate bowl, beat the eggs and salt. Fold the chocolate mixture into the egg mixture. Fold in the flour until just incorporated, being careful not to overmix. Fold in the semisweet chocolate chunks.
Step 4Pour the batter into the prepared pan and bake 45 to 50 minutes, rotating the pan halfway through the baking time. When finished, a skewer inserted in the center will come out looking damp. The edges will be crusty, and the center will feel slightly firm. Do not overbake; the brownies will be dense, soft and fudgy.
Step 5Cool for at least an hour before cutting the brownies into pieces and sprinkling them with cocoa powder.