Heat the oven to 180°C (160° fan) 350°F gas 4.
Butter a loaf tin.
Beat the butter and sugar in a mixing bowl until fluffy.
Gradually beat in the egg yolks until smooth.
Whisk the egg whites with the salt until stiff.
Sift the flour and baking powder into the egg whites and gently stir in with the ground hazelnuts until incorporated.
Mash the bananas with the lemon juice and fold into the butter mixture.
Gradually fold in the egg white mixture until thoroughly blended.
Spoon into the loaf tin and bake for about 45 minutes until golden and risen.
Cool in the tin for 10 minutes, then turn out onto a wire rack to cool.