Preheat oven to 325°F. Lightly grease a 9-inch bundt cake pan, then dust the inside with a mixture of flour and cocoa powder. Turn the pan to coat it evenly and shake out any excess. Set aside.
In a small saucepan, stir the milk and chocolate chips over low heat until it is melted and the mixture is smooth. Set aside and allow the chocolate mixture to cool (but not get cold).
In a mixing bowl, beat the butter and sugar together until well combined.
In a separate bowl, whisk together the flour, cocoa powder, cornstarch, milk powder, baking powder, and salt. Then slowly add the dry ingredients into the creamed butter and sugar. Beat all ingredients together while occasionally scraping the bowl until well combined. It will look sandy.
Pour the vegetable oil into the batter and mix. Then add the eggs one at a time. Continue to scrape the bowl. Next, slowly add the spiced rum and vanilla extract. Followed by the melted chocolate mixture. Continue to mix the batter until it is smooth with no lumps.
Pour the batter into the prepared bundt pan and bake for 45 to 60 minutes, or until you insert a cake tester and it comes out clean. Let the cake cool in the pan on a cooling rack.
In a small saucepan, mix together the butter, sugar, water, cocoa powder, and salt. Bring this to a quick boil. Then turn the heat down and simmer, frequently stirring, for about 5 minutes, or until the syrup has slightly thickened. Remove from the heat and stir in the spiced rum and vanilla extract.
While the cake is still inside the pan, poke holes into the bottom of the cake with a toothpick or skewer. Make sure you poke the holes at least 3/4 of the way down. Pour 1/4 of the rum syrup over the cake. Once the cake absorbs the syrup, pour more, but reserve some syrup for glazing the top. Invert the cake onto a rack. For easy cleanup, place a sheet of parchment paper underneath the rack. Pour the remaining syrup over the top of the cake. Note: you do not need to use all the syrup if you do not desire it.
Let the Chocolate Rum Cake sit for one hour before slicing and serving.