Chocolate Chiffon Cake

Jo always asks me to bake him a nice light chocolate cake for him and I finally get my hand on this recipe from allrecipes.com. Thank you so much for this nice and easy recipe. I halved the recipe from 16 servings to 8 servings and it's enough for one 20 CM cake tin.
*Please note, I only made a plain chiffon cake with no icing coating
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Ingredients:

Baking powder: 22.5g
Boiling water: 90ml
Cake flour: 125g
Sugar: 175g
Baking soda: 3.5g
Salt: 3g
Vegetable oil: 60ml
Eggs, seperated: 4
Vanilla extract: 5ml
Cream of tartar: 0.4g (I used a few drops of lemon juice instead)

Icing:
Butter: 37.5g
Sugar: 120g
Chocolate, melted: 28g
vanilla extract: 4ml
Hot water: 27.5ml
Chopped nuts

Method:

1. In bowl, combine cocoa powder and water until smooth, cool for 20 minutes. In a mixing bowl, combine flour, sugar, baking soda and salt. Add oil, egg yolks, vanilla and cocoa mixture, beat until smooth. In another mixing bowl, beat egg whites and cream of tartar (or lemon juice) until stiff peaks form, gradually fold into egg yolk mixture. Pour into an ungreased 20cm cake tin. Bake on lowest rack at 160 degree celsius for 60-65 minutes or until cake springs back when touched. Invert cake tin to cool, remove cake from pan.

2. For icing melt butter in medium saucepan. Remove from heat, stir in sugar, chocolate and vanilla. Stir in enough water until desired consistency, drizzle over cake. Sprinkle with nuts.