Mix soy milk with margarine (42 g) and sprinkle the yeast on top. Let it rest for 8-10 minutes to activate the yeast.
Add vanilla, sugar, salt and mix. In a large bowl, add both flours and little by little pour the previous mixture. Stir until you get a sticky dough.
Knead for 8-10 minutes until very smooth, but not sticking to your hands.
Grease a large bowl with a Little bit of oil and place the dough inside. Cover the bowl with a kitchen towel and leave the dough to rise in a warm place until doubled in size (about 1 hour).
Scoop the dough out of the bowl and place it on a flour-dusted work surface.
Divide the dough into 10 parts, roll each of them into a ball and transfer to a tray with parchment paper or a silicone mat - leave enough space between each piece. Press each ball lightly.
To prepare the topping: in a bowl, sift the icing sugar with the flour. Add margarine and use your hands to mix until you get a homogeneous dough.
Divide the dough into two parts. To one of them, add vanilla, and to the other, add cacao powder. Mix each of these separately. Refrigerate for 15 minutes.
Divide the vanilla topping into 5 balls, same with the chocolate one. Flatten them with your hands, lightly floured, to form thin disks. Cover the dough balls with the disks and repeat this process until all 10 conchas are complete.
Using a flour dusted knife or cutter, make your preferred pattern on the topping by pressing the knife or cutter into the toppings (you can do just lines or a grid).
Let the conchas rise for 40 min. in a warm place or until doubled in size.
Fifteen minutes before they finish rising, preheat the oven to 170°C (340°F).
Bake on the center rack of the oven for 20-25 min. or until the conchas are lightly golden brown on top and bottom.
Remove from the oven and let them cool completely before eating (they’ll finish baking with the rest of the heat inside the buns).