Preheat oven to 350°F. Grease and line a 9-inch spring form pan.
Soak Chia seeds in water for 15 minutes. Melt butter and chocolate in a heatproof bowl over a pan of simmering water. Allow to cool.
Beat egg whites and 1/3 cup of the sugar until soft peaks form. In a large bowl, beat the remaining sugar and egg yolks until pale and creamy. Pour chocolate mixture over egg yolks and fold. Then fold in the ground hazelnuts and Chia seeds. Finally, fold in the egg whites.
Pour mixture into pan. Bake for approximately 50 minutes. Allow to cool, remove from pan and decorate with icing sugar or melted chocolate.