Start by getting everything ready. Heat your oven to 350°F (175°C). Line the bottoms of two 8-inch cake pans with parchment paper and lightly grease the sides so the cakes release easily later.
In a medium bowl, sift together the cake flour, baking powder, salt and sugar-free cake mix. Once combined, set this aside for now.
In a large mixing bowl, beat the butter until it’s smooth and creamy. Slowly pour in the grape seed oil while mixing, and continue beating until the mixture looks light and well blended.
Add the whole eggs one at a time, mixing well after each addition. Then mix in the egg whites, sour cream, and vanilla extract. Beat until everything is smooth and evenly combined.
Begin adding the dry ingredients to the wet mixture in three parts, alternating with the buttermilk. Mix gently and stop as soon as everything comes together don’t over mix.
Divide the batter evenly between the prepared pans. Place them in the oven and bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.
Remove the cakes from the oven and let them cool in the pans for about 10 minutes. Turn them out onto a wire rack and allow them to cool completely before frosting.
To finish, make the buttercream by beating the butter until fluffy. Add the powdered Swerve, vanilla, and cream, and mix until smooth and creamy. Frost the cooled cake however you like.