Add one stick unsalted butter to a microwave safe bowl and melt in microwave for 30 seconds. Add sugar and mix until well combined. Add vanilla extract, then add eggs, one at a time. Mix until well incorporated. Add in mochiko flour and baking powder, and continue to mix.
Combine evaporated milk and water in a separate bowl. Whisk to blend. Add to mochiko mixture in 2 parts, mixing well between addtions. Next add in coconut milk, whisking until well blended.
Pour mixture into a 9” x 13” pan and bake for one hour at 350F. Promptly remove from oven, and let cool completely. To serve, cut into 1“ x 2” bars.