In a small saucepan, whisk together half cup of the sugar, the cocoa powder and corn syrup with half cup hot water. Bring just to a simmer, stirring occasionally. Add a half-teaspoon of vanilla off the heat and set aside.
Heat the oven to 350 degrees. Butter and lightly flour a 12-cup bundt pan
In a medium bowl, combine the flour, baking powder and salt and set aside.
In the bowl of a stand mixer (or in a large bowl if using a hand mixer), cream the butter with the remaining sugar until light and fluffy.
Whisk in the eggs one at a time
Whisk about a third of the flour mixture into the batter, then a third of the milk. Continue whisking in the flour mixture and milk, alternately and a little at a time, until everything is added and the batter is light and smooth.
Gently fold in the chocolate chips, then divide the batter into thirds. Pour a third of the batter into the prepared bundt pan.
Whisk the chocolate syrup with another third of batter, then pour this into the prepared bundt pan. Pour the remaining third of batter over this
lightly swirl the batters with a wooden skewer or knife to give a "marble" effect and place the pan in the oven
Bake the cake until a toothpick inserted comes out clean and the cake springs back lightly when touched, about 1 hour.
Cool the cake in the pan on a wire rack. Invert the cooled cake onto a serving platter and dust lightly with powdered sugar.