Please read notes about cake size and consistency before starting as well as tips and tricks.
Preheat your oven to 350℉ (180°C). Static setting (NOT convection) is recommended. Butter and lightly flour your cake pan or line it with parchment paper.
Melt the dark chocolate and butter together in a bainmarie, stirring until smooth. Set aside. Alternatively, you can microwave the mixture by placing the butter and chocolate in a microwave-safe bowl, microwaving in 20-second increments, and stirring very well in between.
Separate egg yolks from whites. Whip the yolks with half of the sugar and the orange peel until frothy. Add the liqueur.
Gently blend the melted chocolate into the whipped yolk mixture until smooth. Fold in the almond flour.
Whip the egg whites with the remaining sugar in a separate bowl until stiff peaks form.
Carefully fold the whipped egg whites into the chocolate mixture in two or three batches to maintain a light, airy texture.
Pour the batter into the prepared pan, leveling the surface. Bake in the middle of the oven for about 30 to 35 minutes. The cake should be moist inside; use a toothpick to test the center of the cake at 25 minutes.
Let the cake cool in the pan for 30 minutes before transferring it to a wire rack. Allow it to cool completely for 6 to 7 hours.
To remove the cake from the pan, run a knife along the edges to loosen it. If using a springform pan, release the latch and remove the ring. If using a regular pan, gently tap and tilt to release onto a plate or rack.
Once thoroughly cooled, sprinkle generously with icing sugar just before serving.