Preheat the oven to 170 degrees top/bottom heat.
In a mixing bowl, combine the dry ingredients - flour, starch, powdered sugar, baking powder, vanilla and salt - and mix well.
Then add the liquid ingredients: add the milk, vinegar and oil and mix just until the ingredients are combined. Finally, briefly stir in the sparkling water.
Grease a bundt cake tin and sprinkle with some breadcrumbs or flour to prevent the cake from sticking.
Now pour in about 2/3 of the batter, which is about 700 grams.
Stir the cocoa powder and the tablespoon of vegan milk into the remaining 1/3 and pour it into the tin as well.
To prevent air bubbles from forming or escaping, tap the tin briefly on the work surface.
Bake the cake for about 50 minutes on the second rack from the bottom.
Check with a cake tester or similar, like a thin wooden stick: Insert into the cake - if no batter sticks to it, the cake is baked.
Switch off the oven, open the door and leave the cake to cool in the tin for approx. 10 minutes. Then turn the marble cake out onto a large, flat plate and leave to cool.
Dust with powdered sugar before serving. You could sprinkle the powdered sugar through a fine sieve.