In a small bowl, toss the chocolate chips and a few spoons of Grand Marnier until chips are moistened and shiny. Add the 1 ½ teaspoon flour and toss until evenly coated.
In a medium bowl, lightly combine the eggs, ¼ cup sour cream, and vanilla.
In a large mixing bowl combine the dry ingredients and orange zest and mix on low speed a moment to blend. Add the butter and remaining ¾ cup sour cream. Mix on low speed until the dry ingredients are moistened. Increase to medium high speed for 1 ½ minutes to aerate and develop structure. Scrape down sides. Gradually add egg mixture in 3 batches, beating for 20 seconds after each addition to incorporate the ingredients. Scrape down the sides, and fold in the chocolate chips.
Scrape the batter into a well buttered bundt pan. Bake 55 to 65 minutes or until the cake tests done with a toothpick.
Shortly before the cake is done, prepare the syrup: Heat the sugar, orange juice, and Grand Marnier until the sugar is dissolved. DO NOT BOIL. As soon as the cake comes out of the oven, place the pan on a rack, poke the top all over with a wire tester, and brush on ½ the syrup. Cool in the pan for 10 minutes, then invert onto a serving plate. Brush with the remaining syrup and cool completely before wrapping airtight.
This cake can be made several days ahead and refrigerated, wrapped well. I like to glaze it with a Chocolate Cream Ganache before serving.