Veggie Cake Onigiri

To season the rice: 

In a bowl, season the cooked sushi rice with seasoned rice vinegar or a dash of white vinegar, salt, and sugar. Sprinkle in the sesame seeds and mix to combine well. Set it aside.
Shaping onigiri with a mold:

Soak the onigiri mold in a bowl of water to prevent the rice from sticking to the mold.
Place the mold on a flat surface and fill it halfway with the seasoned sushi rice. Place a veggie cake (or filling of choice) on top of the rice.
Fill the rest of the mold with rice and cover with the lid. Firmly press down to ensure the rice holds its shape.
Wrap the nori sheet around the rice ball and transfer it to a plate. Repeat the steps with the remaining ingredients.