2/3 c  Self raising flour
1/4 ts Bicarbonate of soda
1/3 c  Castor sugar
30 g  Butter
1/3 c  Water
1    Egg
PEPPERMINT FILLING
2 1/4 c  Icing sugar
1 tb Oil
2 tb Milk approx
Peppermint essence
CHOCOLATE TOPPING
125 g  Dark chocolate chopped
90 g  Unsalted butter

Grease a 25x30cm Swiss roll pan, line base with paper,grease paper.
Sift flour, cocoa and soda into small bowl of electric mixer, add
sugar, butter and water; beat on medium speed for two minutes. Add
egg, beat until combined. Spread mixture into prepared pan. Bake in
moderate oven for about 20 minutes; cool in pan.

Spread base with filling, refrigerate until set, spread with topping,
refrigerate until set before cutting.

Peppermint filling: Combine sifted icing sugar and oil in snall heat
proof bowl, stir in enough milk to make a stiff paste; flavour with
essence. Stir over hot water until spreadable.

Chocolate topping: Melt chocolate and butter over hot water.

Some other chocolate toppings to experiment with:

Rich Chocolate Glace Icing 90 g dark chocolate, chopped 1 tsp
Vegetale oil 1/4 cup (60ml) water 2 cups (320g) icing sugar mixture
approx

Combine chocolate, oil and water in a medium saucepan, stir over a
low heat until chocolate is melted. Gradually beat in enough sifted
icing sugar to give a smooth spreading cosistency.