Gradually add the water to the shiratamako rice flour, dissolving it and creating a watery white liquid.
Then add this liquid to the shiroan paste (which is a sweet, white kidney bean paste) in a small pot, and mix very well with a rubber spatula until the liquid is completely incorporated. Place the pot on the stove, and set it on medium-low heat. Keep kneading the dough with the spatula until most of the water has been absorbed, and it no longer has a shine. It should still have a paste-like consistency and hold together well, but feel and look slightly dryer than before. You don’t want to cook it so long that it begins to brown, and remove it immediately from the heat if you notice it browning. On the other hand, if you do not cook it long enough it will be much too sticky to handle.
Once it has reached this stage, remove it from the heat, and spread the paste evenly along the insides of the pot, for it to cool. Spreading it around ½ to 1 cm thick will allow the paste to cool without drying out too much.
Once the nerikiri has cooled enough to touch and not burn your hands, scrape it off from the pot with a spatula, and roll it into a ball. If it crumbles and cracks when you are trying to shape it, it has been overcooked, and if it sticks to your hands and is very difficult to shape, it has been undercooked. In both of these instances, I would recommend making a new batch. My first batch did not turn out well at all, but you will quickly get used to the dough and what it should look like. It should feel like a slightly denser version of Play-Doh.
Knead the nerikiri together with the pink food coloring, and if you are going to add a happy face to your brains, tear off a small piece of the nerikiri and knead it together with some cocoa powder.