Slice the sweet potato into 1cm thick slices, peel, and soak in water for 5 to 10 minutes to get rid of any bitterness.
Roast the nuts lightly in a toaster oven on the weak setting (500 W). If the nuts are whole, chop them up. (The nuts in the photo are macadamias.)
Chop up the A. white chocolate, and break up the B. white chocolate with your hands.
Wrap the soaked sweet potato slices in plastic film, and microwave at 500 W for about 3 minutes and 30 seconds until tender.
Put the microwaved sweet potato and the B. white chocolate in a bowl with the butter, and mash up while still hot. Add the B. milk also and mix well.
Put the A. butter in another heatproof bowl. Microwave for 30 seconds at 500 W, take out and cream together with the sugar.
Add the beaten egg into the Step 6 mixture in several batches, mixing well. Preheat the oven to 170°C.
Combine the cake flour and baking powder and sift into the the Step 7 mixture. Fold in with a rubber spatula, and when it's more or less mixed, add the A. milk and mix again.
Add the mashed sweet potato from Step 5, the chopped nuts and the A. chocolate and mix.
Put the batter into a kitchen parchment paper lined mold, and spread it out so that it's level.
Bake in a preheated 170°C oven for about 30 minutes. Take it out of the mold, and cool on a rack.
Cut into pieces once it's cooled down. (If any are left over, put the pieces in a plastic bag and freeze. Re-heat in the microwave and they'll become fluffy and delicious and again.)
Story Behind this Recipe
Sweet potatoes and white chocolate go well together. I wanted to make a sweet with nuts, so I tried making this. I made it so that it would have a moist finish, for my husband who is not fond of sweet potatoes. He ate it saying "this is the most delicious sweet potato cake I've ever had".
It's a heavy batter, so please spread it around well once it's been put into the mold.