Butter a 9x13 inch pan. Set aside.
Preheat oven to 350.
In a large bowl or bowl of a stand mixer, whisk together mochiko, sugar, baking soda and cocoa powder. Set aside.
In a small, heavy-bottomed saucepan, melt the butter and both chips together until melted and smooth, stirring frequently. Allow to cool slightly.
Meanwhile, in a medium bowl, whisk together the milk, eggs, and vanilla.
Whisk in the chocolate mixture until well combined.
Pour the wet ingredients into dry ingredients and whisk until smooth.
Pour the mixture into the prepared pan and bake for 50 minutes or until the center of the pan is set and when the tip of thin knife is inserted into the middle of the pan it comes back basically clean with no raw batter.
Cool completely, in pan on a wire rack. Sprinkle with coconut, slice.