Preheat oven to 350°F (175°C). Line the bottom of an 8-inch springform pan with parchment and lightly grease the sides.
Melt dark chocolate and butter together until smooth; cool slightly.
Beat egg yolks with half the sugar until paler and slightly thickened.
Mix the melted chocolate mixture into the yolks until glossy.
Fold in ground almonds and a pinch of salt.
Whip egg whites until foamy, then gradually add the remaining sugar and beat to glossy stiff peaks.
Gently fold egg whites into the chocolate batter in 2–3 additions.
Pour into the pan, smooth the top, and bake about 28–35 minutes, until set with moist crumbs.
Cool completely before slicing. Dust with powdered sugar if desired.
