Preheat oven to 325 degrees, grease (very well) and sugar bundt pan with butter wrapper.
In mixer, cream sugar & butter. Add flour, cornstarch, baking powder, dry milk, salt and 3 tb of oil combing until evenly mixed (but not overly mixed). The mixture will look like find crumbs.
In a separate bowl, whisk eggs, milk, rum, vanilla extract, and the rest of the canola oil. Slowly add it to the dry mixture and mix until combined. The batter will be thin.
Pour into the sugared and well-greased bundt pan and bake for 55-60 minutes. The cake should be well cooked to support the rum syrup (as my mum would say).
Allow the cake to cool for 10 minutes before inverting onto a rack. Let it cool for another 20-30 minutes.
While the cake is cooling, start the rum syrup. In a saucepan over medium heat: heat butter, water, sugar, and salt until the sugar dissolves, stirring occasionally. Allow the mixture to reduce slightly and then remove from heat and stir in the rum.
Wash and dry the bundt pan, then place over the cooled cake and invert the cake back into the pan. Poke holes into the cake and pour the syrup slowly over the cake. Let it soak for at least 8 hours (or overnight) and in the morning, invert the cake back onto a serving platter.