Grease eight 6-oz (175 mL) ramekins; line bottoms with parchment paper.
Set ramekins aside.
In large bowl over saucepan of hot (not boiling) water, melt chocolate with butter until smooth; set aside.
In another heatproof bowl over saucepan of gently simmering water, whisk egg yolks with 1/2 cup (125 mL) of the sugar; cook, whisking constantly, until light and thickened, about 8 minutes.
Fold one-quarter into chocolate mixture; fold in remaining yolk mixture.
Set aside.
In large bowl, beat egg whites until soft peaks form; beat in remaining sugar, 2 tbsp (25 mL) at a time, until stiff peaks form. Fold cocoa and one-third of the egg whites into chocolate mixture; fold in remaining egg whites.
Spoon into prepared ramekins.
Bake in centre of 350°F (180°C) oven until puffed and edges are set, about 20 minutes. (Make-ahead: Cover and refrigerate for up to 1 day.)
raspberry sauce: In blender or food processor, purée together raspberries, water, sugar and lemon juice until smooth; press through fine sieve to remove seeds. (Make-ahead: Refrigerate in airtight container for up to 1 week.)
Invert cakes onto dessert plates; remove paper.
Garnish each plate with raspberry sauce; top cake with scoop of raspberry sorbet.