Wash the sweet potatoes.
Then cook them with their skins on for 20 minutes in salted water.
Peel them and then mash them.
Preheat the oven to 180C/350F/Gas mark 4. butter and lightly flour a 23cm cake tin.
Mix together the sweet potatoes, ginger, 2 whole eggs, 2 egg yolks, butter, flour, sugar, vanilla essence, milk and rum.
Whisk egg whites until stiff, then fold into the sweet potato mixture.
Pour into the prepared cake tin and bake for 30 minutes.
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