Start by preheating the oven to 350°F. Grease three 6-inch round cake pans and line their bottoms with parchment paper circles. This helps the cakes release easily from the pans. Lightly grease the parchment paper as well to prevent sticking.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Mixing these ingredients first ensures the leavening agents distribute evenly. Set this bowl aside while you prepare the wet ingredients.
Using a mixer, beat the softened butter on high speed until light and fluffy. Add the sugar and continue mixing until the mixture becomes creamy and smooth. This step helps incorporate air into the batter for a softer cake.
Mix in the egg whites and vanilla bean paste until fully combined. Then add the sour cream and blend again until smooth. These ingredients help give the cake its moist texture and rich vanilla flavor.
Reduce the mixer speed to low and slowly add the dry ingredients to the wet mixture. Pour in the milk while mixing gently. Mix just until everything is combined, avoiding overmixing.
Divide the batter evenly among the prepared cake pans. Bake for about 18–20 minutes or until a toothpick inserted into the center comes out clean. Once baked, allow the cakes to cool completely in their pans.
Wash and slice the strawberries before placing them in a food processor. Blend them briefly until they form a puree with small visible bits. This puree will add natural flavor and color to the frosting.
In a mixing bowl, beat the cream cheese and butter until light and fluffy. Add powdered sugar gradually while mixing. Pour in the strawberry puree and mix until smooth and creamy.
Place one cake layer on a serving plate and spread frosting evenly on top. Add the second layer and repeat the process. Finish by placing the third layer on top and frosting the entire cake smoothly.