Divide the cake batter in half. For this I actually weigh out the batter and divide it in half.
Mix half of the dough with the cocoa powder and once that's incoporated, add the mik and mix it in as well.
Place the vanilla batter in one pastry bag and the chocolate batter in a different one.
Grease your bundt pan with oil or soft butter. You can paint it on or use a piece of kitchen paper to spread it around. Flour the mould. After distributing the flour around the inside of the mould, turn it upside down and pat it to remove the excess.
Make a ring at the base of your bundt pan with one flavour. On top of it, make a ring with the other flavour. Eventually, you will see that the dough will start to spread out as you pipe more and more. Continue until you have used up all the batter. If you are using a cake pan, do this just at the centre of the pan making a circle within a circle. If you're using a rectangular pan, do this as a line down the centre.
Bake the cake at 180°C/350°F for 40min-1hour or until a knife comes out clean. It's important to not overcook it to prevent it from drying out. If you have an electric oven, check the cake at the 30-minute mark. If it's golden enough on top, change the heat source to only the bottom.
Unmould the cake while it's still hot.