Preheat Oven & Prepare Pan : Preheat your oven to 180°C (350°F). Grease an 8-inch round cake pan and line the bottom with parchment paper.
Melt Chocolate and Butter : In a heatproof bowl over simmering water, melt the dark chocolate and butter together until smooth. Set aside to cool slightly.
Whisk Wet Ingredients : In a large bowl, whisk the sugar and eggs until light and creamy. Add the cooled chocolate mixture, orange zest, orange juice, and optional liqueur. Mix thoroughly.
Combine Dry Ingredients : Sift the flour, almond meal, baking powder, and salt into a separate bowl. Gradually fold the dry ingredients into the wet mixture using a spatula until fully combined.
Pour Batter into Pan : Transfer the batter into the prepared pan and smooth the top. Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted comes out clean.
Cool and Garnish : Let the cake cool in the pan for 10 minutes before transferring it to a wire rack. Garnish with fresh orange slices, powdered sugar, or whipped cream before serving.