Heat the oven to 350°F (175°C). Grease a 9-by-13-inch baking dish with the softened butter.
Place the thawed grated cassava in a large bowl. If it has released a large amount of water while thawing, gently press out only the excess liquid; the cassava should still be moist.
Add the coconut milk, condensed milk, evaporated milk, eggs, sugar, melted butter, vanilla, and salt. Stir well until the mixture is evenly combined and no streaks of egg remain.
Pour the mixture into the prepared dish and smooth the surface. Sprinkle with shredded coconut, if using.
Bake for 55 to 65 minutes, until the top is golden in places, the edges are set, and the center no longer looks wet or liquid when the dish is gently moved. A toothpick may come out moist because cassava cake has a soft, pudding-like texture.
Set the dish on a rack and cool for at least 30 minutes before slicing. For cleaner squares and a firmer texture, cool completely, then chill before cutting. Serve slightly warm or chilled.