1. Preheat the oven to 350°. With a pastry brush, spread 1 tablespoon of the softened butter over the bottom and sides of a 9-by-5-by-3-inch loaf pan.
2. Drop the sweet potatoes into enough lightly salted boiling water to cover them completely and cook briskly until they are soft. Drain the potatoes thoroughly, then force the potatoes and banana through a ricer or food mill set over a deep bowl.
3. Beat in the remaining 4 tablespoons of softened butter, add the eggs, and mix well. Add the sugar, corn syrup, milk, evaporated milk, vanilla, nutmeg, cinnamon and raisins, and beat until the batter is well blended. Then pour it into the prepared pan.
4. Bake in the middle of the oven for 1½ hours, or until a cake tester or tooth-pick inserted in the center comes out clean and the top is golden brown. Let the cake cool in the pan for about 5 minutes, and then turn it out onto a wire rack to cool completely.
5. Gateau de patate has a somewhat pudding like texture. Traditionally it is sliced and served as a dessert or with tea. You may if you like accompany the cake with whipped cream, sweetened and flavored with vanilla or rum or coquimol.
