1. Add sugar and vanilla extract to egg yolk and mix, then add vegetable oil and milk and mix. 2. Sift the soft flour and mix. Divide the meringue into 3 portions and mix. 3. Transfer 2/5 of the dough to another bowl, mix cocoa powder and warm milk, put it into the dough, and mix. 4. Put the white dough in the pan and flatten it. Squeeze the chocolate dough using a piping bag, stir it up and down, stir it on both sides, and bake it in an oven preheated to 170 degrees for 20~22 minutes. 5. Remove the parchment paper, apply ganache on the white part, roll it using parchment paper, and cool it in the refrigerator.