Preheat the oven to 325°F. Generously grease a square glass 8-inch baking dish with butter.
In a medium bowl, whisk together the eggs, milk, melted butter, stevia, cinnamon, cocoa powder, ginger, and salt.
Whisk in the almond flour and baking soda.
Pour the batter into the prepared pan. Bake until the cake is puffed and a toothpick inserted in the center comes out clean, about 20 minutes.
Cool the cake in the pan on a cooling rack for 20 minutes, then cut it into nine squares and serve.
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Notes
Occasionally, baking soda can cause a reaction in baked goods that results in an ammonia smell. If you're concerned about it happening, use ½ tablespoon of fresh, non-expired baking powder (gluten-free if needed) instead of baking soda.
You can use a granulated sweetener instead of stevia. If the batter seems too thick, add an extra tablespoon of milk.
I don't recommend using ginger powder. The flavor will suffer. If you must, you can use ¼ teaspoon, but I highly recommend waiting with this recipe until you have fresh ginger.
This might be somewhat unusual, but I like to add a tablespoon of cocoa powder. It gives the cake a deeper color and an additional layer of rich flavor. This is useful because I don't use brown sugar or molasses.
If using baking soda, it's best to use natural cacao powder (not treated with alkali). Its acidity will help activate the baking soda, ensuring a fluffy cake. If using baking powder, it's fine to use cocoa powder treated with alkali.
Once completely cool, you can wrap the cake in cling wrap, then in a layer of foil, and store it in the fridge. It keeps well for about 5 days. You can also wrap and freeze it, or freeze individual slices in freezer bags.