Add brown sugar, coconut sugar, and water into a small pot over medium-low heat. Mix and cook until all sugar dissolves. Let it cool down.
Whisk together the brown flour, potato starch, buckwheat flour, tapioca flour, baking powder, baking soda, and salt in a bowl.
Add the cooled sugar water mixture into the dry ingredients. Mix to combine. Then add the melted oil and almond milk.
Line the bottom of a loaf pan (Mine is 22×11.5 cm). Pour the mixture into the pan. Gently drop it on the counter a few times to remove the air bubbles. Let it sit for 20 minutes.
Add 3 rice-cups of water (about 540 ml) to the rice cooker (I use this old-fashioned one). Let it cook until the water is boiling, then carefully place the pan into the cooker to steam for about 30 minutes. You can also steam it on the stove. Using a toothpick to test if it's cooked.
Remove the pan from the cooker. Sprinkle some white sesame seeds on top. Let it cool a little before lift it out from the pan. Cut and enjoy!