• Preheat your oven to 325 degrees F. Spray two 8-inch cake pans or three- 6" cake pans with non-stick spray. Set aside.
• Into the bowl of an electric mixer with the paddle attachment, beat butter and oil together for 3-4 minutes until they are completely unified.
• With the mixer on low, gradually pour your  sugar into the mixture and beat on medium to high speed until light and fluffy (about 3 minutes). Scrape down the sides of the bowl.
• With the mixer on low speed add egg whites in one at a time. Allow each to incorporate before the next addition. Next, add in the vanilla, lemon zest, orange zest and lemon juice. Increase the speed of your mixer to high and beat until mixture is smooth and light in colour.
• While your sugar mixture is mixing, sift together flour, baking powder, and salt into a medium bowl.
• With the mixer on low speed, add one third of the flour mixture into the bowl and mix until just combined. Scrape down the bowl and add one half of the orange juice and again mix until just combined. Repeat this process until everything has been added to the mixing bowl. Begin and end with the flour mixture. Take care not over mix or your cake will become quite dense.
• Distribute cake batter evenly between two 8-inch cake pans or three 6-inch pans. I like to use a kitchen scale to ensure batter is divided equally.
• Bake for 28-35 minutes, or until knife comes out clean. Each oven is slightly different, so I suggest setting your timer to 28 minutes and watching carefully from there!
• Cool on cooling rack for 10 minutes before inverting and removing cakes from can pan to cool completely.
• Cool completely before frosting. 

For the Blackberry Swiss Meringue Buttercream

• Begin by preparing the blackberry puree. This needs to cool before it can be added in to the Swiss meringue buttercream.In a medium saucepan, combine blackberries, lemon juice, and sugar.
• Over medium-high heat, bring the mixture to a gentle boil. Stir continuously to avoid burning. Reduce heat to medium and allow to simmer for about 6-8 minutes. The blackberries will become soft and easy to mash.
• Mash blackberries and simmer for 1 more minute.
• Remove from heat and pass mixture through a strainer to remove any large blackberry pieces. (Note, I like keeping small pieces of blackberry in my puree because I think it gives a nice texture - you can see the size in the photos above)
• Allow to cool completely before adding to buttercream.
• While blackberry puree is cooling, prepare the Swiss Meringue Buttercream.
• Wipe down your mixing bowl, whisk and paddle attachments with a small amount of white vinegar.
• Create a double boiler on your stove by filling a medium saucepan with a couple inches of water. Do not let the water touch the base of the bowl. Put your egg whites and sugar in your bowl and place on top of the saucepan. Give them a whisk to combine.
• Cook your egg white/sugar mixture over medium heat stirring constantly until the mixture reaches 160 degrees F on a candy thermometer or until you can no longer feel any sugar granules between your fingertips. The mixture will be hot.
• Transfer your egg white/sugar mixture to the bowl of your electric stand mixer with the whisk attachment in place and whisk on high until the outside of your mixing bowl is no longer hot and you’ve achieved stiff peaks in your meringue. This can take between 8-12 minutes.
• Switch to the paddle attachment. Turn your mixer to low speed. Add in your cubed butter (2 tablespoon in size) one at a time, allowing each to be incorporated before the next addition.
• Scrape down the sides of your bowl, turn mixer to low speed and add in your vanilla and blackberry puree and lemon juice. Make sure to add in slowly or buttercream may separate. If buttercream does appear to separate you can whip on high to combine again. 
• Turn your mixer up to medium to high speed and mix for about 1 minutes until buttercream is completely incorporated, silky, and smooth.