In a large bowl, whisk together the eggs, milk, melted butter, vanilla, and stevia.
Add the almond flour, mixing it in with a rubber spatula and then whisking until the batter is smooth. Whisk in the kosher salt and baking powder.
Divide the batter evenly between the two pans. To ensure the cake layers are the same size, it’s best to use your kitchen scales and weigh the batter as you pour it into the pans. I weighed each cake at 360 grams.
Bake the cakes in the middle of the oven until slightly puffed and set and a toothpick inserted in their center comes out clean, 15-18 minutes. The tops of the cakes won't brown even when they are fully baked.
Remove the cakes from the oven and place them on a wire rack. Let them cool for 10 minutes in the pans, then invert them directly onto the wire rack. Let them cool completely for at least 30 more minutes.
While the cakes are cooling, prepare the frosting:
Place the softened unsalted butter, heavy cream, cocoa powder, vanilla, and stevia in a medium bowl. Using a handheld electric mixer, beat on low to incorporate, then beat on medium for about 1 minute until the frosting is light and fluffy. You can also mix the frosting by hand until light and fluffy.
Assemble and frost the cake:
Place one of the cake layers on a cake plate, flat side up. To keep the plate clean, you can line the plate with strips of wax paper to protect the plate (let them overhang), then pull them from underneath the cake once you’re done frosting it.
Place ⅓ cup of frosting in the middle of the cake and use an offset spatula to spread the frosting over the top of the cake.
Gently place the second cake layer on top of the first one, flat side up. There’s no need to press on it. Place another ⅓ cup of frosting on top of this second cake, and use the spatula to spread it all over the top of the cake.
Use the remaining frosting to frost the sides of the cake.
Once frosted, you can cut the cake and serve it. You can also refrigerate it, covered, until ready to serve. Take it out of the fridge 2 hours before you plan on serving it.
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Notes
It's important to use nonstick cake pans and grease them well. Butter works better than nonstick spray. I replace my baking pans once a year, so they are always brand new and truly nonstick.
If your pans are not brand new and super nonstick, line their bottoms with a parchment circle after greasing them and grease the parchment too.
A granulated sweetener in the frosting instead of stevia will feel grainy, so please use a powdered sweetener and add a bit more heavy cream (start with ¼ cup) to get the right consistency.
I recommend using Dutch-processed cocoa powder (cocoa treated with alkali) in the frosting. It's milder and less acidic than natural cocoa powder. This is not mandatory, but I tried both options, and I like the frosting better with the Dutched cocoa.
Make sure the cake layers are completely cool before frosting the cake or the frosting will melt as you spread it on the warm layers. You can speed up the process by placing the cakes, still on the cooling rack, in the fridge.
You can store this cake in the fridge, covered, for up to five days. I don't own a cake dome, so I cover the cake plate with an inverted salad bowl. Do remember to take the cake out of the fridge at least an hour, and preferably two hours, before you plan on enjoying it. Frosting is always best at room temperature.