Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Sift mochiko, sugar, cocoa powder, and baking soda together in a bowl.
Place butter in a microwave-safe bowl. Heat in the microwave until melted, about 1 minute. Stir in chocolate chips until smooth and creamy.
Beat coconut milk, evaporated milk, and eggs together in a bowl. Add vanilla extract. Add chocolate-butter mixture and stir until incorporated. Add to the flour mixture slowly, whisking by hand until well incorporated and no lumps remain. Pour into the prepared pan.
Bake in the preheated oven until set, about 55 minutes.
Cool completely before cutting, about 1 hour. Cut with a plastic knife; mochi sticks to metal.
