Preheat the oven to 170°C (340°F) using top and bottom heat.
Thoroughly mix the dry ingredients—flour, starch, powdered sugar, baking powder, vanilla, and salt—in a mixing bowl.
Add the milk, vinegar, and oil, mixing only until combined, then briefly stir in the sparkling water.
Grease a Bundt pan and dust it with breadcrumbs or flour to prevent the cake from sticking.
Pour approximately two-thirds of the batter (about 700 grams) into the prepared pan.
Stir the cocoa powder and vegan milk into the remaining third of the batter and pour it into the pan.
Tap the pan on the work surface to allow any air bubbles to escape.
Bake the cake on the second rack from the bottom for approximately 50 minutes.
Check for doneness by inserting a wooden skewer; the cake is finished if the skewer comes out clean.
Turn off the oven, open the door, and let the cake cool in the pan for 10 minutes before inverting it onto a large, flat plate to cool completely.
Dust the cake with powdered sugar through a fine sieve before serving.
