Preheat the oven to 180℃ (160℃ fan/350℉) and line a 20 cm (8-inch) square baking tin with parchment paper. Leave some overhang for easy removal.
Place the dark chocolate and butter in a microwave-safe bowl. Microwave in 20-second increments, stirring between each interval, until just melted. Sift in the cocoa powder and whisk to combine. Set aside to cool slightly (lukewarm is fine, but it shouldn't be hot).
In a large mixing bowl, use your fingers to massage the orange zest into the sugar until fragrant (this releases the natural oils for a stronger orange flavour).
Add the eggs to the bowl and, using an electric mixer, beat together with the sugar until the mixture is very pale, the sugar has mostly dissolved, and the beaters leave thick ribbons when lifted (about 3–5 minutes).
Add the slightly cooled chocolate-butter mixture, vanilla, and orange liqueur (or orange juice) and beat on low speed until just combined.
Use a rubber spatula to fold in the almond meal and salt, then fold in the chocolate chips.
Scrape the brownie batter into the lined baking tin. Spread into an even layer, making sure to push it into the corners.
Bake on the middle rack for 30-35 minutes. For super gooey brownies, start checking around the 25-minute mark. A toothpick inserted into the centre should come out with thick, moist crumbs, but no wet batter. For firmer brownies, bake closer to 35 minutes, until a toothpick comes out mostly clean. The brownies will continue cooking once they come out of the oven, so be careful not to overbake them.
Allow to cool fully in the tin before removing and slicing. These are especially delicious served warm with vanilla ice cream!