Combine boiling water and oats in a Thermoserver or bowl and allow to soak for up to 1 hour.
Preheat oven to 170ºC. Grease a square cake pan and line base with baking paper.
Measure butter, sugars and egg into TM bowl and mix 30 sec/speed 4. Scrape sides of bowl and mix 30 sec/speed 4.
Add remaining cake ingredients to TM bowl including 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.
To make icing, measure all ingredients into TM 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. KEEP A VERY CLOSE EYE on the icing under the grill so it does not burn!
I recently made a healthier version of this cake substituting all sugar (both in the cake batter and in the icing) with equal quantities coconut sugar, the flour in the cake for plain spelt flour and the evaporated milk in the icing for an equal quantity of pouring cream. Although I didn't feel it was quite as good as the original, I took the cake to church for morning tea and two elderly ladies raved about how gorgeous it was - one even said it was the best cake she's ever eaten...so it had to be pretty good then! ;)