Preheat oven to 180C (top and bottom heat). Line the base and sides of a 20 cm square cake pan, at least 5 cm deep, with a large square of wilted banana leaf (no need to grease), shiny side up, tucking its corners to fit.
Combine grated tapioca with coconut milk, sugar, salt, butter and egg yolks in a heavy-bottomed pan. Set over medium-low heat and stir constantly, scraping pan base and sides with a whisk, until mixture thickens into a creamy but pourable custard.
Scrape batter into lined pan. Bake on the oven’s lowest shelf for 40 to 45 minutes. Then, scatter cold butter bits over the bengka, and move pan to a high shelf in the upper third of the oven. Bake for 10 to 15 minutes more, until bengka is well browned and a skewer inserted into its centre emerges sticky but clean, with no raw batter on it.
Let kueh cool to at least lukewarm before unmoulding.
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