Preheat oven to gas mark 4, 180 °C, fan 160° C, 350° F.
Cream together the butter and sugar until pale and fluffy.
Whisk in 1 whole egg, separate the other one and add the yolk.
Fold in the flour. Spoon into a 500g / 1lb lined loaf tin.
In a separate bowl whisk remaining egg white until it forms soft peaks then gently fold in the demerera sugar and coconut.
Gently spread egg white mixture over the cake mixture and bake for 30 minutes until cake is cooked and top is crunchy. Serve when ready.
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