Line a cupcake pan with papers and preheat oven to 325*F.
Using a whisk attachment on a stand mixer, beat the egg whites and cream of tartar on high speed until soft peaks form. Set aside or transfer to another bowl.
Using the paddle attachment, cream butter, erythritol, and stevia on medium speed until fluffy. Add egg yolks one at a time until incorporated, then add sour cream and vanilla.
Combine the almond flour, cocoa powder, and baking powder in a small bowl, stir to mix.
Using a spatula, add about one-third of the egg whites to the butter mixture, and stir until combined. Add half of the almond flour mixture, fold to combine. Add half the remaining egg whites and fold to combine. Add the remaining nut mixture, fold. Add the last of the egg whites, fold to combine.
Using an ice cream scoop or large spoon, fill cupcake wrappers to the top with the batter. Bake for 20-25 minutes or until the cakes spring back slightly when touched. Cool in the pan on a wire rack.
Cream Cheese Icing
Beat cream cheese on medium speed with paddle attachment until smooth and creamy. Switch to whisk attachment and slowly pour in whipping cream while beating. Add the vanilla. When combined, add erythritol, one tablespoon at a time, until desired level of sweetness is achieved (I used about 1.5 T). Raise speed to high for two minutes to whip the mixture.
Pipe onto cooled cupcakes. Store in the refrigerator.
*For greater depth of flavor, I like to use a combination of cocoa powders, but you can certainly just use whatever you like best.