Mix all ingredients together. Pour into a 11x15" dish. Bake at 350F for 1 hour; the top will bubbly, golden brown and poofy. Cool completely (it will deflate) before slicing (it's easiest with a plastic knife so that the mochi doesn't stick to the knife) and serving.
You can store leftovers at room temperature for a few days or in the fridge. If you refrigerate it, however, the mochi gets a little hard and I like it best reheated (I prefer toasting or microwaving).
From Aunty Lily (there are a bunch of very similar ones that you can Google as well; not sure where Aunty originally got the recipe from).
During my most recent batches, the butter (which rises to the surface of the mochi during baking) has bubbled over a bit in a 9x13" pan, so I suggest either using an 11x15" pan or using a 9x13" pan with a baking sheet on the rack below the pan to catch any drips. Another thing to keep in mind: an 11x15" pan yields mochi with a higher crispy crunchy top to chewy mochi ratio, whereas a 9x13" pan yields mochi with a thicker middle and less of the topping. Your choice, but I love more top (chances are, if you're into muffin tops, you'd like the thinner, crispier mochi, too).