First, make the cake. Position a rack in the center of the oven and preheat the oven to 350°F. Generously spray a 9- or 10-inch round springform cake pan with cooking spray and place it in the middle of a half sheet pan lined with parchment paper.
In a medium bowl, whisk together the sugar, salt, and eggs. Pour in the coconut milk and evaporated milk, and whisk to combine. Finally, whisk in the cassava.
Pour the batter into the prepared pan and use an offset spatula to smooth its top if necessary.
Bake the cake. Bake for 45 to 50 minutes, or until the sides of the cake are set and the center of the cake bounces back when gently pressed. A skewer inserted into the center of the cake should come out with a few crumbs attached. Cool slightly on a wire rack while you make the custard topping.
Next, make the custard topping. Position a rack as close as possible to the broiler's heat source and preheat for 10 minutes.
In a medium bowl, whisk together the coconut milk, evaporated milk, condensed milk, egg, and vanilla. Pour the filling over the baked cassava cake, then drizzle the melted butter on top.
Broil the cake. Broil for 15 to 20 minutes, or until the custard starts to sizzle and becomes mottled and molten. Cool completely on a wire rack before slicing.
Serve and store. Run an offset spatula around the outer edge of the cake before unmolding the cake pan's sides. Slice into wedges and serve slightly warmed, or at room temperature. The cassava cake can be stored at room temperature, loosely covered in plastic wrap, for up to 1 day. After that, place in an airtight container and refrigerate for up to 5 days.