To prepare the frosting, place the cashews, syrup, non-dairy milk or water, salt, and extract in a high-speed blender or food processor. Blend or process until the mixture is mostly smooth; in a high-speed blender, this will be about 2 minutes once you get to high speed, and in a food processor it should take about 4 minutes, with a few breaks to scrape the bowl down. Then, with the machine running, drizzle in the coconut oil. Continue blending till the frosting is smooth and emulsified. Transfer it to an airtight container to chill for at least 2 hours. It will keep in the fridge for up to 4 days.
Preheat your oven to 350F. Grease and flour a 9-inch, springform cake pan (I like to place a piece of round parchment in the bottom as well, to prevent any chance of the cake sticking).
Sift the flour into a large mixing bowl, then stir it together with the salt, baking soda, baking powder, and sugar.
In a separate mixing bowl, stir together the banana, yogurt, oil, non-dairy milk, lemon, and vanilla. Add these wet ingredients to the dry ingredients and fold them together until the batter is evenly mixed and no flour remains at the bottom. A few lumps in the batter is OK.
Transfer the cake to the oven and bake for 35-40 minutes, or until the top is golden and firm to the touch and a toothpick inserted into the center emerges clean (depending on your oven, it could take up to 45-50 minutes, but I recommend checking it after 35 minutes to see how far along the cake is). Remove the cake from the oven and the springform pan, then allow it to cool completely before frosting.
Frost the cake, reserving any extra frosting for dipping pieces of fresh fruit, or for another frosted treat. Sprinkle the cake with the nuts, then slice and serve.
*You can also use your favorite gluten-free, all purpose flour combination.