Combine boiling water and oats in a bowl and allow to soak for up to 1 hour.
Preheat oven to 170ºC. Grease a 20cm square cake pan and line base with baking paper.
Measure butter, sugars, eggs, cinnamon, salt, baking soda and baking powder, and vanilla into the mixing bowl and mix 30 sec/speed 6. Scrape sides of bowl and mix 30 sec/speed 4.
Add flour and soaked oats and mix 10 sec/speed 4. Scrape sides of bowl and mix 10 sec/speed 4.
Pour into tin and bake 50-60 minutes checking after 50 minutes.
To make icing, measure all ingredients into the mixing bowl and combine 5 sec/Reverse/speed 4. Scrape sides of bowl and combine 5 sec/Reverse/speed 4. Spread icing over hot cake and place under hot grill until bubbling and golden (about 5 minutes). KEEP A VERY CLOSE EYE on the icing under the grill so it does not burn!
Cool in tin before removing it and serving.