Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving a 2-inch overhang on two sides. Butter the parchment and any exposed pan sides.

Combine butter and chocolate in a medium saucepan over low heat. Stir constantly until completely melted and smooth, about 4-5 minutes. Remove from heat and let cool slightly, 2-3 minutes.

Whisk sugar into the warm chocolate mixture until glossy. Add eggs one at a time, whisking well after each addition. Stir in vanilla, orange zest, orange juice, and orange liqueur if using. The batter should look thick and satiny.

Add flour, cocoa powder, and salt. Switch to a spatula and fold gently until just combined and no dry streaks remain. Do not overmix - this keeps the brownies fudgy.

Scrape batter into prepared pan and smooth the top. Bake until a toothpick inserted in the center comes out with moist crumbs attached, 32-36 minutes. The center should look set but still slightly soft. Cool completely in the pan on a wire rack, at least 2 hours.

Use the parchment overhang to lift brownies from the pan. Cut into 16 squares with a sharp knife, wiping clean between cuts. Sprinkle with flaky salt if desired.