Preheat the oven to 180C (top and bottom heat). Line the base and sides of a 20cm square cake pan (at least 5cm deep) with a large square of wilted banana leaf, shiny side up.
Combine the grated tapioca with the coconut milk, sugar, salt, butter and egg yolks in a heavy-bottomed pan. Set over medium-low heat and stir constantly, scraping the base and sides, until it thickens into a creamy but pourable custard.
Scrape the batter into the lined pan. Bake on the lowest shelf for 40-45 minutes. Scatter the cold butter bits over the top and move to a high shelf; bake 10-15 minutes more until well browned and a skewer comes out sticky but clean.
Let cool to at least lukewarm before unmoulding. Slice with a plastic knife to serve.
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