Preheat the oven to 375°F/195°C. Grease and flour a 9-inch/23-centimeter Bundt pan and set aside.
Place the milk, eggs and vanilla extract in a medium bowl and lightly whisk, just to combine.
Sift both flours and the salt directly into the bowl of an electric mixer with the paddle attachment in place, then add the sugar and mix on low speed for 30 seconds. Add the butter and half the egg mixture and continue to mix until the dry ingredients are incorporated. Increase the speed to medium and beat for 1 minute. Scrape down the sides of the bowl, then gradually add the remaining egg mixture, in two batches, making sure the first batch is fully incorporated before adding the next. Don’t worry if your batter looks slightly split; it’s due to the large proportion of eggs in the batter, but it won’t affect the final result.
Scrape down the sides of the bowl and divide the mixture among two bowls. Add the ground cardamom to one bowl and fold to combine. Warm the 1 1/2 tablespoons milk in a small saucepan, then place in a small bowl with the coffee granules and cocoa powder. Stir until the coffee dissolves and the consistency is that of thick but pourable milk. Combine this with the cake mixture in the second bowl.
Spoon the two mixtures into the prepared pan in four alternate blocks, two of each color. Then, use a skewer or small knife to make a zigzag-shaped swirl once through the mix to create a marble effect. Do not be tempted to overdo the swirling as you will lose the effect of the marbling.
Bake for 40–45 minutes, or until a skewer inserted into the center of the cake comes out clean. Remove from the oven and set aside for 10 minutes. The cake tends to dome in the oven, so if you want a perfectly flat base (the top will become the bottom once it’s inverted), just slice off the top to flatten it out before turning the cake out onto a wire rack to cool completely.