Throw the lanka and water in a sauce pan and boil at medium heat until the lanka starts to break down and get thick and jammy. Add more or less of the date syrup depending on how sweet you want the jam. At this stage it will still be stringy so run it through the food processor to get it nice and smooth. Let it cool. Place about 4 tablespoons in a separate cup as you will use this reserved jam to swirl.
Make the Mochi Batter
Mix all the wet ingredients in a bowl until smooth.
Mix all the dry ingredients in a separate bowl.
Mix the dry ingredients in to the wet mixture until no lumps appear and pour in a greased 9x13 baking pan. Dot the reserved Lanka Jam throughout the batter and then use a tip of a knife to swirl through.
Bake for 45-50 until it's nice golden and springy. Cool fully in the pan and then turn over to a cutting board and cut into squares. ⠀⠀⠀⠀⠀⠀⠀⠀⠀
Notes
Note this recipe is party size. It makes about 30ish squares of butter mochi depending on how you cut it. You can also replace the lanka with other fruit or jam. ⠀⠀⠀⠀⠀⠀⠀⠀⠀