Heat oven to 350 degrees F.
Grease an 8x8 glass baking dish by dipping a pastry brush into the melted butter and spreading a thin layer to evenly coat.
Whisk the cassava, the remaining melted butter, the macapuno, half a cup each of the coconut milk and condensed milk, and the salt together in a large mixing bowl.
Pour the mixture into the prepared pan and move to the center rack of the oven and bake for 45 minutes.
Meanwhile, whisk the egg yolks with the remaining coconut milk and condensed milk in a medium bowl and set aside.
When the 45 minutes are up, carefully remove the cake form the oven and set on your cooktop, a cooling rack, or trivet. Pour the topping mixture evenly over the cake then return it to the oven and cook for another 20 minutes.
Remove the cake to a cooling rack and boost the oven to broil.
When your oven has hit full broil, place the cake back in the oven, still on the center rack, and broil until the top begins to bubble and caramelize, about 2 minutes. It will burn quickly, so keep an eye on it.
Remove the cake from the oven and cool, on the rack, to room temperature before serving. If you'd like to serve it chilled, cover the room temp cake with plastic wrap and refrigerate for at least an hour.