Sugar free vanilla cake with a sugar free swiss meringue buttercream.
Preheat the oven to 325 degrees F. Prepare two 8-inch or three 6-inch round cake pans with non-stick spray, parchment paper on the bottom of the pan, and another coat of spray. Set aside.
In the bowl of a stand mixer, fitted with the paddle attachment, beat the butter, oil, and monkfruit on high until fluffy and smooth, about 5 minutes.
Add the eggs and the egg whites, one at a time and beating well between each addition, to the bowl and beat on medium until well incorporated. It may look curdled, don't stress.
Add the vanilla extract and beat on low until incorporated. Scrape down the sides of the bowl and mix on low for another 30 seconds. Fold in the sour cream.
Evenly distribute the cake batter among the two 8-inch or three 6-inch pans and bake for 35 to 40 minutes, until toothpick comes out with just a few moist crumbs on it.
In the bowl of your electric mixer, or any heat proof bowl, combine the egg whites and monkfruit. Whisk until combined.
Heat about 1 to 2 inches of water in a medium pot over medium heat. Place the metal mixing bowl, or other heat proof bowl, over the pot to create a double boiler. Stir continuously, until the mixture because smooth, light in color, frothy and reaches 160 - 170 degrees F on a candy thermometer.
Once the mixture has reached the desired consistency, use a whisk attachment to beat the mixture until you reach medium-stiff peaks, about 10 minutes. It takes a bit of time, so be patient. You'll know it's ready for the next step when you touch the outside of the bowl with your inner wrist and it's room temperature. If it's still warm, keep beating until there's no heat coming off the bowl.
Once you've reached medium-stiff peaks, change to the paddle attachment and then gradually add the butter in small squares, followed by the vanilla. Continue to beat for about 3 to 5 minutes until smooth and silky.
Place the first cake layer, top side up. Add about 1 cup of frosting. Spread over the cake layer as evenly as possible. Make sure to get eye level with the cake to ensure the frosting is spread evenly over the cake.
Place second cake layer on top and apply a thin coat of frosting around the entire cake to lock in the crumbs. Freeze for about 10 minutes.