Preheat oven to 350°F Butter a 10-inch Bundt pan or two 9x5-inch loaf pans.
In a bowl, gradually whisk milk into cocoa powder to make a smooth paste; whisk in yogurt and set aside. In another bowl, combine flour, baking powder, baking soda and salt. In a large bowl, using an elecric mixer, beat butter with sugar until light and fluffy; beat in eggs and vanilla. Using a wooden spoon, stir in flour mixture, alternately with milk mixture, making 3 additions of flour and 2 additions of milk. Fold in chocolate chips.
Spread into prepared pan, smoothing top. Bake for 60-70 minutes or until a tester inserted in center comes out clean with a few moist crumbs clinging to it. Let cool in pan on a rack for 15 mintues. Invert onto rack, remove pan and let cool completely. Serve dusted with icing sugar.