Preheat oven to 350 degrees F.
In a large bowl, mix the unsalted butter and Xylitol until smooth.
Add the eggs, sour cream, vanilla extract, and mix until well combined.
Stir in the almond flour, coconut flour, baking powder, and salt.
Using an ice cream scoop, scoop the batter evenly into your muffin tin and bake for 27 minutes or until fully cooked through.
To make the buttercream, mix the butter, powdered sweetener, and vanilla extract for 3-4 minutes.
Add the milk and mix for an additional minute or two on high speed.
Using a star tip and piping bag, frost the cooled cupcakes and enjoy!