Chop the dark chocolate into pieces, put it in a heat resistant bowl with the butter, and melt everything on top of a pot with hot water (double boiler) until smooth. Let it cool slightly.
Separate the yolks from the whites of 3 eggs. In a bowl, whisk the yolks with half of the sugar and some lemon zest until the mixture becomes creamy and pale (about 4 minutes with an electric whisk).
Add the melted chocolate to the yolk mixture and mix to combine.
Add almond flour (or finely blended raw almonds, peeled or unpeeled) and whisk until well incorporated.
In a clean, dry bowl, whisk the egg whites. Once they start to foam, add the remaining sugar and beat until stiff peaks form (about 5 minutes).
Gently fold half of the egg whites into the chocolate mixture using a spatula, then fold in the remaining half until fully combined.
Butter a 7-inch pan and then line with parchment paper so the paper sticks. Pour in the batter.
Bake in a preheated oven at 350°F (175°C) for about 35 minutes, then let the cake cool completely.
Optionally brush the surface with 1–2 tablespoons of rum, dust with icing sugar, and serve. The cake stays fresh for several days and is intensely chocolatey and moist.
