Add 2/3 cup palm sugar (about 140 grams) to a pan, turn the heat to low, and add 2 tablespoons water (about 30 ml). Cook for about 5 minutes until the sugar turns a nice caramel color. Add 2/3 cup coconut milk (about 160 ml), gently stir to combine, then turn off the heat and pour the mixture into a bowl to cool slightly.

Sift 3/4 cup tapioca starch (about 110 grams) and 1 tablespoon rice flour (about 8 grams) into the warm sugar mixture (about 86–104°F / 30–40°C). Use a spatula to mix in one direction for about 5–10 minutes until the batter is smooth and well combined.

Crack 3 large eggs into a bowl and gently beat them in one direction, being careful not to create too many bubbles. Add the eggs to the batter, then add 1 tablespoon vegetable oil. Mix everything together with a spatula. Cover the bowl with plastic wrap or a towel and let the batter rest for 20–25 minutes.

Preheat the oven to 340°F (170°C) for about 10 minutes to stabilize the temperature. Grease the baking pan with a little butter to prevent sticking. Place the pan in the oven to heat at 340°F (170°C) for 5 minutes.

Add 1 teaspoon baking powder (about 5 grams) and 1 teaspoon baking soda (about 5 grams) to the batter. Gently mix in one direction until well combined. Remove the hot pan from the oven and pour the batter in, filling it about 2/3 full to allow room for the cake to rise. Bake at 320°F (160°C) for 50 minutes. After baking, transfer the cake to a rack, invert it to remove from the pan, and let it cool. Once cooled, cut into pieces and serve with extra coconut milk if desired.