Preheat your oven to 325°F (162°C). Spray a 12-cup bundt pan very well with cooking spray.

In a large bowl, combine the flour, dry milk powder, cornstarch, baking powder and salt. Set this aside.

In a large bowl with an electric mixer or stand mixer, cream the butter and sugar at high speed until light and fluffy. Add the dry flour mix to this. It will resemble fine crumbs.

In the bowl that just had the flour mix, whisk together the oil, milk, eggs, rum, and vanilla extract. Add this to the crumb mixture and beat until well combined. The batter will be quite thin.

Pour the batter into the prepared bundt pan and bake for 48-60 minutes, or until a toothpick inserted in the middle comes out clean.

When there are about 15 minutes remaining on the timer, start the butter rum syrup. In a large saucepan over medium heat, heat the butter, water, sugar and salt until the sugar has dissolved, stirring occasionally. Take the saucepan off the heat and stir in the rum (it'll bubble).

Let the cake cool in the pan for about 10 minutes and then invert onto a wire rack. Wash and dry the bundt pan and place the cake back into the pan. Poke holes into the cake (I used a meat thermometer to do this) and pour the syrup slowly over the cake. Let soak for 1-2 hours and then invert the cake back onto a serving platter. Cover and store at room temperature for up to 5 days.