Steps

1. Preheat the oven to 180˚C/Gas 4.
2. Sift the flour, matcha powder and baking powder together. Beat the butter until soft.
3. Fold the flour mixture, eggs and sugar into the butter and beat.
4. Spoon the mixture into cake cases. Fill the cases to about two-thirds to prevent cake explosion.
5. Bake for 16 minutes or until the cake tops spring back when lightly touched with the back of a fork.
6. Take out and leave to cool on a wire rack.
7. As the cakes are cooling, remove the skin from the sweet potatoes and cut into bite-sized pieces. Place the pieces in cold water.
8. Drain, put the sweet potato pieces into a pan, and add 100ml milk. Bring to a boil, turn the heat down, then add the rest of the milk, stirring slowly.
9. Break up the sweet potato pieces with the back of a spatula, and remove the pan from the heat.
10. Press the sweet potatoes through a small-meshed sieve with a spatula (you can cheat and use a blender).
11. Add 30g of caster sugar to the double cream and whip for approximately 7 minutes, until the cream is thick enough to stand up in peaks.
12. Add the whipped cream to the sweet potato paste and mix well. Add two or three drops of vanilla essence.
13. Scoop into a piping bag and pipe over the green tea cupcakes. Decorate with whole boiled azuki beans.