Place a rack just below the center of the oven and preheat to 325 degrees. Grease and flour Bundt pan very well. Set aside.
In a medium-sized mixing bowl, whisk together the flour, cornstarch, baking powder, powdered milk and kosher salt then set aside.
In a separate large mixing bowl or the bowl of a stand mixer, cream together the sugar, butter, coconut oil and vanilla until light and fluffy. Beat in 1 egg at a time on medium speed, making certain to scrape down the sides of the bowl. Ensure that each egg is fully incorporated before adding the next. Stir the milk and rum together in a 2-c liquid measuring cup then set aside.
Add about ⅓ of the flour mixture to the wet ingredients and mix gently until combined then add ½ of the milk mixture and stir again until well combined. Repeat this process, ending with the dry ingredients. Mix until fully combined and remember to scrape down sides of the bowl. Add the pecans, folding them gently into the batter.
Pour the batter into the prepared Bundt pan and bake for 50–60 minutes, or until a toothpick inserted in the middle of the cake comes out with a few moist crumbs attached. Remove from the oven and cool on a rack for 10 minutes.
After cooling slightly, invert the cake onto the cooling rack and allow it to cool an additional 20–30 minutes.
Meanwhile, wash and dry the Bundt pan then prepare the rum syrup. For the syrup, place the butter, water, sugar and salt into a small saucepan and set over medium heat.
When the mixture begins to bubble, reduce the heat to medium-low and simmer for 5 minutes, stirring constantly. Remove from heat and stir in the rum.
Now for the fun part! Place the cooled cake back into the Bundt pan and use a bamboo skewer or long toothpick to generously poke holes throughout the cake. Slowly pour the hot rum syrup evenly over the cake, saturating it entirely. Cover the pan with aluminum foil and allow the cake to soak up the syrup for at least 8 hours at room temperature. Invert cake once again onto a cake plate and serve.
One Last Thing
Use butter rather than cooking spray to generously grease a Bundt pan. It does a much better job preventing your cake from sticking to the pan.