Bring water to a boil in a saucepan. Add mochi into the boiling water and simmer until soft. When done, strain excess water away from the mochi.
Place the butter on the drained mochi to melt. Add flour, sugar, and salt. Mix briskly.
Once mixed, add in the yolk. Combine and mix briskly. Line a Tupperware container with a sheet of plastic wrap, and dust the surface of the wrap with katakuriko.
Using a spatula, transfer the buttered mochi into the tupperware box. Coat the surface with katakuriko, and smooth it out using the backside of a spoon.
Story Behind this Recipe
I usually pound mochi rice to make it, but I tried using cut mochi this time to simplify things a bit. Since I've discovered I could use cut mochi to make spongy buttered mochi, here's the recipe for you to try!
I recommend you soften the store-bought mochi decently before mixing because it's quite sticky. Use homemade mochi or Japanese rice mochi for the best results.
It's really soft when it's done, so please chill in the fridge (or in the freezer if you're rushing) before slicing. Take care to coat the sliced surfaces of the mochi so they don't stick together!