Place the ground almonds, sugar, flour, baking powder and salt in the bowl of an electric mixer fitted with a whisk attachment and whisk to combine. Add the eggwhite and whisk for 35 seconds, then add the butter and whisk for 1 minute. Rest for 30 minutes in the refrigerator.
Pour the batter into the tin.
Once cooled, grate the cake, then spread out the crumbs on a tray and place in a food dehydrator* or an oven at 80°C (176°F) for 2 hours or until dry and crisp.
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