Preheat oven to 325 degrees F. Grease an 8-inch square glass baking dish with butter. Be generous when greasing to ensure the cake releases easily from the pan.
In a large mixing bowl, whisk together the eggs, melted butter, stevia, and vanilla extract.
Gradually whisk in the almond flour and cocoa powder.
Finally, whisk in the kosher salt and the baking soda. The batter will be quite thick.
Pour the batter into the prepared baking dish and smooth the top out with a rubber spatula. Bake until the center is set and a toothpick inserted in it comes out clean, 18-20 minutes.
Cool the cake, in the pan, completely, about 1 hour, then frost if you wish.
To make the frosting, place all the frosting ingredients in a medium bowl and whisk by hand. Or beat with an electric handheld mixer on low speed until smooth and fluffy, 1-2 minutes.
Nutrition info for an unfrosted slice: Calories 206, Fat 19g, Saturated Fat 8g, Sodium 166mg, Carbohydrate 5g, Fiber 3g, Sugars 1g, Protein 6g.A granulated sweetener in the frosting will feel grainy - so please use a powdered sweetener. If using a powdered sweetener in the frosting instead of stevia, it's possible that you would need to add a bit more heavy cream (or even water) to get the right consistency.