Preheat the oven to 350°F. Generously grease the bottom and sides of an 8-inch nonstick round cake pan with unsalted butter.
In a medium bowl, using a hand whisk, whisk together the eggs, coconut milk, stevia, and vanilla.
Whisk in the coconut flour. Keep whisking until the batter becomes smooth and free of lumps. If the batter seems very thick, add 1-2 tablespoons of water.
Using a rubber spatula, stir in the kosher salt and baking powder.
Using the same rubber spatula, transfer the batter to the prepared pan. Smooth the top out.
Bake until the edges are golden brown and pull away from the pan, and a toothpick inserted in the center comes out clean, about 20 minutes (see notes). The cake will not brown on the top.
Place the pan on a cooling rack. Let the cake cool for 15 minutes, then invert it, bottom side down, onto the cooling rack and allow it to cool for 15 more minutes.
If using, dust the cake with a powdered sweetener, then slice and serve.
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Notes
Make sure your cake pan is truly nonstick. This is super important when baking with coconut flour. I purchase a new nonstick pan once a year.
Oven temperatures vary. This cake needs 20 minutes in my oven but might need a longer time in yours. It's ready when a toothpick inserted in the center comes out clean.
You can replace the stevia with any granulated sweetener to equal ½ to ¾ cup of sugar. You won't be able to replace it with a liquid sweetener like maple syrup or honey.
When working with canned coconut milk, it's very important to make it completely smooth before adding it to the batter. If it's not thoroughly combined, that would introduce too much liquid and not enough fat into the batter. It's not enough to shake the milk in the can. You should stir it well once you've opened the can. The best way to do that is to pour the entire can contents into a bowl (use a spatula to get everything out of the can) and use a hand whisk.
For a 9-inch pan, you'll need to increase the ingredients by 25-30%: 5 large eggs, 1 ¼ cups coconut milk, granulated sweetener or stevia to equal ⅔ cup of sugar, and 105 grams of coconut flour. The vanilla, salt, baking powder and powdered sweetener can stay the same.
You can keep the leftovers in an airtight container in the fridge for up to 5 days. You can also freeze individual slices in freezer bags. Defrost them overnight or in the microwave at 50% power.