Preheat oven to 325°F (165°C). Line a 12-cup muffin tin with cupcake liners.
In a large bowl, whisk together the almond flour, coconut flour, erythritol, baking powder, and salt.
In a separate bowl, whisk the eggs, melted butter, heavy cream, and vanilla extract. Pour the wet ingredients into the dry and stir until a smooth batter forms. Let it rest for 2 minutes.
Divide the batter evenly among 10 of the cupcake liners, filling each about two-thirds full.
Bake for 20 to 25 minutes, until the tops spring back when lightly touched and a toothpick comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Make the buttercream by beating the butter and cream cheese together until smooth. Add the powdered erythritol, vanilla extract, and heavy cream.
Beat on medium-high speed for 3 minutes until light and fluffy. If the frosting is too soft, refrigerate it for 10 minutes.
Pipe or spread the buttercream generously onto the fully cooled cupcakes. Refrigerate any leftovers.
