Preheat the oven to 350 degrees F (175 degrees C). Butter a 12x17-inch rimmed baking sheet.
Beat condensed milk, coconut milk, and melted butter together in a large mixing bowl until combined. Add eggs, 1 at a time, and beat until combined. Add macapuno, 1 jar at a time, and beat until combined. Gradually beat in mochiko flour. Beat in brown sugar; beat in vanilla extract. Pour batter onto the prepared baking sheet.
Bake in the preheated oven until top is golden brown and sides are slightly dark brown, 45 to 50 minutes.
Allow to cool, about 20 minutes, before loosening the sides and cutting into squares.