Preheat oven to 375F, and butter a 22 cm-diameter cake tin and line with baking paper.
Melt chocolate and butter in a heatproof bowl over a saucepan of simmering water, stirring occasionally until smooth. Remove from heat, and stir in buttermilk, cocoa, and egg yolks.
Meanwhile, whisk egg whites until frothy in an electric mixer, then, while whisking, gradually add the sugar and whisk until glossy soft peaks form. Fold in chocolate mixture in batches along with the almond flour, pour into prepared tin and bake until cake rises and cracks at the top (25-30 minutes). Cool in tin, gently pressing on top as it cools so it falls back evenly, then remove from tin, dust with cocoa (and maybe some whipped cream!) and serve. Cake will keep for 2-3 days stored at room temperature in an airtight container.