To make the batter, in a large mixing bowl, beat the eggs. Add the evaporated milk and avocado oil, then whisk until smooth. Add the dark brown sugar and mix until dissolved. Sift in the flour and salt, then mix until well combined. Add the instant yeast and mix again, then cover the bowl.
To ferment the batter, let the batter ferment for about 1 hour, or until bubbles form on the surface. To speed up fermentation, preheat the oven to its lowest setting for 1 minute, then turn it off. Place a pot of boiling water on the bottom rack and the bowl of batter on the middle rack. Close the oven door to create a warm, humid environment that helps the yeast work more efficiently.
To add the leavening, sift the baking powder and baking soda into the fermented batter, then whisk until fully incorporated.
To prepare the steamer, line an 8-inch bamboo steamer or heatproof container with parchment paper, trimming away any excess. Pour the batter into the lined steamer.
To steam the cake, wrap the steamer lid with a clean kitchen towel to catch condensation. In a large pot over high heat, bring water to a boil. Reduce the heat to medium-high, place the steamer over the pot, cover, and steam for about 30 minutes, or until a toothpick inserted into the center comes out clean.
To cool and serve, turn off the heat and carefully remove the cake from the steamer. Let it cool slightly before slicing and serving.