Step 1In a small saucepan, whisk together one-half cup of the sugar, the cocoa powder and corn syrup with one-half cup hot water. Bring just to a simmer, stirring occasionally. Add a half-teaspoon of vanilla off the heat and set aside.
Step 2Heat the oven to 350 degrees. Butter and lightly flour a 12-cup bundt pan.
Step 3In 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 until thoroughly incorporated, then whisk in the remaining vanilla.
Step 4Whisk 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.
Step 5Gently fold in the chocolate chips, then divide the batter into thirds. Pour a third of the batter into the prepared bundt pan.
Step 6Whisk 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.
Step 7Bake 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.