1. Preheat oven to 325ºF. Mist a 9-by-5-inch loaf pan with cooking spray. In a small bowl, whisk together flour, baking powder and salt. In a separate small bowl, whisk yogurt, milk and vanilla.
2. In a large bowl, using an electric mixer on medium-high speed, beat butter with sugar until light and fluffy, about 3 minutes. Beat in eggs one at a time, beating well after each and scraping down side of bowl. Reduce speed to low. Add half of flour mixture, then yogurt mixture, then remaining flour mixture, beating after each addition until nearly combined. Remove 1/3 of batter (about 1 cup) to a medium bowl.
3. Pour 1/4 cup boiling water into a small bowl. Whisk in cocoa and coffee powder until smooth; let cool slightly. Add to batter in medium bowl; stir to combine.
4. Alternate spoonfuls of vanilla and chocolate batters in loaf pan. Draw a knife through batters for a marbled effect. Bake cake until a toothpick inserted in center comes out clean, about 50 minutes. Cool on rack for 15 minutes, then remove from pan and cool completely.
5. Place cream in a microwavable cup or bowl. Microwave until hot, about 25 seconds. Add chocolate chips. Let stand for 2 minutes; stir until smooth. Let cool to warm room temperature (icing should still be pourable). Drizzle icing over cake, letting it drip down sides. Top with sprinkles, if desired. Let stand until icing has set.