Preheat oven to 325°F. Grease two 8-inch round cake pans and line them with parchment paper.

In a large bowl, beat butter, monk fruit sweetener, and oil until fluffy and pale.

Add eggs and egg whites one at a time, mixing well after each addition.

Alternate adding dry flour mix and buttermilk, starting and ending with flour.

Stir in vanilla extract and fold in sour cream.

Divide batter into pans and bake for 35–40 minutes until a toothpick comes out clean.

Cool for 10 minutes in pans, then transfer to wire racks.