Preheat oven to 325F. Grease three 6" cake pans and set aside (see note).
In the bowl of a stand mixer, combine butter, coconut oil, sugar, baking powder, baking soda, and salt. Mix until light and fluffy. Scrape down the sides of the bowl.
With the mixer running on low, add the egg whites one at a time. Add vanilla bean paste (see note) and mix to combine.
With the mixer running on low, alternate additions of flour and buttermilk. Add ⅓ of the flour, followed by ⅓ of the buttermilk. Repeat for remaining flour and buttermilk, allowing each addition to incorporate before adding the next.
Divide the batter between the prepared pans (see note).
Bake for about 30 minutes, turning the cakes around at the halfway point to help ensure an even bake. A toothpick inserted in the center of the cake should come out clean. Allow the cakes to cool completely on a wire rack.
For Buttercream:
In the bowl of a stand mixer, cream butter and sifted powdered sugar until smooth.
Add vanilla extract and cream. The buttercream will be on the thin side until we add the berries.
In the bowl of a food processor, pulse the berries until they turn into a fine powder. You will want to make sure that everything you use is COMPLETELY dry to keep the powder from clumping up.
You can process all the berries together and put them all in the buttercream for a mixed berry buttercream. I chose to process the berries separately, divide the buttercream evenly between three bowls, and have three different flavors of buttercream (see note on flavoring buttercream).
Frost cooled vanilla cake layers with buttercream. If storing in the refrigerator, allow the buttercream to warm before serving.Enjoy!
Notes
Note on cake flour and coconut oil: These ingredients help lend the cake it's super soft and fluffy texture. I don't suggest substitutions for these!Note on buttermilk: If you don't have buttermilk, you can make a quick substitution with milk (2% milkfat or greater) and vinegar or lemon juice. For this recipe, pour 1 tablespoon lemon juice or vinegar into a liquid measure, then add enough milk to come up to the 1 cup line. Let sit for 5 minutes, then it's ready for use!Note on vanilla bean paste: You know me, it's in everything! Delicious and more economical than whole vanilla beans, this ingredient is my go-to for an extra punch of flavor. You can find it online [here]. If you prefer to substitute, you can use an equal measure of vanilla extract.Note on freeze-dried berries: Make sure to use *freeze-dried* berries, as they are free of moisture and will process into a powder. If you can't find them at your local grocery store, they are available from Amazon ([strawberries], [raspberries], and [blueberries]).Note on flavoring buttercream: If you choose to process the different berries separately, like I did, I suggest starting with the strawberries (lightest in color) and ending with the blueberries (darkest in color). That way, you can reuse the food processor bowl for each berry type without having to wash it out. You also won't end up with dark remnant pieces of blueberry in your lighter strawberry buttercream.Note on cake pans: This recipe is for a 6" triple-layer cake. If you're looking to get some 6" pans for yourself, these are the ones that I use [here]. If you don't have three pans, the batter will still be good at room temperature for about three hours if you need to bake them individually. See note on using a kitchen scale--it's much easier to divide the batter equally when you have one!Also...I've made this recipe in both 8" pans and 6" pans. If you prefer to make an 8" triple-layer cake, double the cake recipe as written here. Bake time will be about 40 minutes. For the buttercream, increase the amount of butter, powdered sugar, vanilla, and cream by ⅓ and use the same amount of freeze-dried berries. The buttercream is quite flavorful, so I don't think additional berries are necessary with this slight increase in volume.Note on kitchen scale: To ensure I have the same amount of batter in each pan, I use my kitchen scale (link here) and weigh the pans. It's hard to eyeball the amount when you're using three pans, especially if you don't have three and you're baking the cake layers individually.