To make lemon curd, place butter in a medium saucepan; strain beaten egg into pan. Add remaining ingredients; stir over low heat, without boiling, 10 minutes or until mixture thickly coats the back of a spoon. Transfer curd to a medium heatproof bowl; refrigerate until cold.
Preheat oven to 160°C/325°F. Insert the base of a 24cm (9½-inch) springform pan upside down to make cake easier to remove. Grease and line base with baking paper.
Spread nuts, in a single layer, on an oven tray; roast, uncovered, 12 minutes or until skins begin to split. Cool. Chop nuts finely.
Beat egg whites and ¼ cup of the sugar in a small bowl with an electric mixer until firm peaks form. Add remaining sugar; beat on high speed 5 minutes or until sugar is dissolved. Fold in chocolate and nuts. Spread mixture into pan.
Bake meringue about 40 minutes. Cool in pan.
Beat the cream in a small bowl until soft peaks form; fold half of the cream into lemon curd. Spoon curd mixture onto meringue. Refrigerate 1 hour until firm.
Spoon remaining whipped cream onto cake; top with blueberries. Serve immediately.