The Best Ever Chocolate Fudge Self Saucing Pudding

This is my other half's favourite - he is not much of a dessert eater but he'd eat all of this given half the chance!

batter:
3/4 cup sugar
1 egg
1 cup flour
1/4 cup cocoa

2 tsp baking powder

1/4 tsp salt

1/2 cup milk

1/4 cup butter, melted
1 tsp vanilla extract

sauce:
1/2 cup packed brown sugar
1/4 cup cocoa
1 1/4 cup hot water

Preheat oven to 180°C. Combine sugar, egg, flour, cocoa, baking powder and salt. Stir in milk, butter and vanilla; beat until smooth.  Pour batter into ungreased 9-inch square baking pan. Stir together the brown sugar and cocoa; sprinkle mixture evenly over batter. Pour hot water gently over top; do not stir. Bake for 35 to 40 minutes or until center is almost set. Remove from oven; let stand 15 minutes.