Bring 2.5 cups of water in a small pot to a boil. Add the Chinese brown brick sugar (which is also called slab sugar), and gently stir to dissolve.*If you are using ginger, add the ginger slices to water and bring to a boil. Let the water and ginger boil together for 3 minutes before adding the sugar. Once the sugar is fully dissolved, remove the pot from heat. Set aside and let cool to room temperature. Then remove the ginger (if using).
Sift the mochiko flour into a mixing bowl. Gradually pour in the water-sugar mixture. Make sure the water-sugar mixture is at room temperature before you mix it with the mochiko flour.
Stir until the mixture is completely smooth, no lumps. I use a whisk for this step (a hand mixer is also good).
Stir in the oil until everything is evenly mixed. Set aside.
Lightly grease (with neutral oil) the pan/bowls that you will use to steam the gau.You can steam this gau recipe in a single pan, look for a 6 or 7-inch pan. Or steam the gau in multiple bowls (which is what I like to do). Use what you have, no need to buy a new pan for this recipe.Just make sure that it is round, for good luck! For the recipe pictured in this post, I steamed the gau in two noodle soup-sized bowls. Set up your steamer (I use one like this) and bring the water to a boil. While you wait for the water to boil, pour the gau mixture into the greased pan/bowls. Gently cover with foil.
Once the water boils, steam the gau for 2-3 hours over medium-high heat. The actual steam time depends on the size of the pans/bowls that you use. When dividing this recipe between two bowls, I steamed for a total of 3 hours. I recommend checking for doneness at the 2 hour mark. Poke a toothpick in the center of the gau (please be careful of the heat/steam!) If the toothpick comes out clean, the gau is done cooking. Carefully remove the gau from the steaming pot. Then gently press a Chinese red date in the center and top with a sprinkle of roasted sesame seeds ^_^