Mix the milk with the sugar and sprinkle the yeast on top. Let it sit for 8-10 minutes to activate the yeast.
Add the margarine or oil, vanilla, salt and mix. In a large bowl, add the flour and little by little pour the previous mixture. Mix until you get a sticky dough.
Knead for 8-10 minutes until very smooth, but not sticky to your hands.
Transfer the dough to a large bowl and cover with a kitchen towel. Let the dough rise in a warm place until doubled in size (approx. 1 hour).
Remove the dough from the bowl and place it on a work surface previously dusted with a little bit of flour.
Roll the dough out (to the thickness of your preference; I went for about 5mm thick). Cut the dough into a heart shape, either with a paper template, as shown in the video, or with a cookie cutter.
Transfer to a tray lined with parchment paper or a silicone mat – leave enough space between each piece-.
To prepare the topping: in a bowl, add the icing sugar, flour, margarine, vanilla, food coloring (add the latter little by little until desired hue), and mix with your hands until you get a homogeneous dough. Add a tablespoon of milk only if the dough requires it, that is, it breaks easily and feels slightly dry.
Divide the vanilla topping into 9 parts. Flatten them with a tortilla press or a plate or even just with your hands (it's much easier if you place them between two pieces of plastic to make them flattening easier), until you get thin disks. Cut each disk into the same heart shape and size. Place on top of the dough hearts and press lightly. Repeat this process until all the conchas are complete.
Using a flour dusted knife or concha 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).
Cover the conchas with a kitchen towel and let them rise for 20-30 min. in a warm place.
Fifteen minutes before they finish rising, preheat the oven to 345 °F (170°C).
Bake on the center rack of the oven for 17-19 min. or until the conchas are lightly golden brown on top and bottom. NOTE: The time may vary depending on the thickness of the dough and the size of the heart. This time (17-19 min) is approximate for a dough thickness of 5 mm and a heart size of 14x12 cm.
Remove from the oven and let them cool completely before eating (they’ll finish baking with the rest of the heat inside the conchas).