Heat oven to 180°C.

In a heavy saucepan, melt butter, 50g sugar and the chocolates over low heat, stirring constantly. Cool until the mixture is lukewarm.

In a large bowl, beat egg yolks until thick and lemon-colored, about 3 minutes. Beat in vanilla. Gradually beat in pecans and chocolate mixture.

In a small bowl and with clean beaters, beat egg whites on medium speed until soft peaks form. Gradually add remaining sugar, 1 tablespoon at a time, beating on high speed until stiff peaks form. Stir a small amount of whites into the chocolate mixture. Fold in remaining whites.

Pour into a greased 23cm springform tin. Place on a baking sheet and bake for 40-50 minutes or until a toothpick inserted in the centre comes out with a few moist crumbs.

Cool on a wire rack for 20 minutes.

Carefully run a knife around the edge of the tin to loosen; remove sides of tin and cool completely. Garnish with strawberries and mint if desired.