- With the rack in the middle position, preheat the oven to 350°F (180°C). Generously butter and flour a 10 to 12-cup (2.5 to 3 litre) Bundt pan.
- In a small pot, heat the milk with the butter until melted. Let cool.
- In a bowl, combine the flour, baking powder and salt.
- In another bowl, whisk the eggs, sugar and vanilla with an electric mixer until the mixture is pale and doubled in volume, about 5 minutes. With the machine running on low speed, whisk in the dry ingredients alternating with the cooled milk mixture until smooth.
- Spread out two-thirds of the batter (about 3 cups/750 ml) in the prepared Bundt pan. Fold the cocoa and coffee powders into the remaining batter. Gently pour the mocha batter into the middle of the vanilla batter. Using a small spoon, gently swirl the two batters in small circular movements to create a marbled effect.
- Bake for 50 minutes or until a toothpick inserted in the centre of the cake comes out clean. Let cool for 10 minutes. Turn the cake out onto a wire rack set over a baking sheet. Let cool completely.
- In a bowl, dissolve the coffee powder in the water. Whisk in the icing sugar. If necessary, add a small amount of water or icing sugar to obtain a pouring consistency. Pour over the cooled cake, letting the icing run down the sides.