Preheat the oven to 350° F. Prepare 4 8-inch round cake pans with parchment paper circles and cooking spray.

In a medium mixing bowl with a whisk, combine the flour, baking powder, and salt. Set aside.

In a separate large mixing bowl, beat the butter, oil, and sugar until smooth and fluffy.

Add the eggs, one at a time, mixing each until combined. Then add in the vanilla extract, and mix until combined.

Add the dry mixture, alternating with the milk, until just combined and the batter is smooth.

Divide the batter evenly into the 4 cake pans. Bake for about 20-25 minutes or until a toothpick inserted in the center comes out clean.

Allow the cakes to cool completely while making the filling and buttercream.

Combine the berries, sugar, and lemon juice in a blender or food processor, and blend until pureed.

Transfer to a saucepan over medium heat. Allow the mixture to simmer for about 10 minutes or until it thickens.

Transfer to a small bowl and chill until ready to use.

In a large mixing bowl, beat the room-temperature butter until smooth and creamy.

Gradually add the powdered sugar until completely combined.

Add the blackberry filling 1 tablespoon at a time until the desired color and flavor are reached.

Level the cakes and stack them with a small layer of buttercream and blackberry filling in between each cake layer.

Frost the entire cake with the remaining buttercream. Slice, and enjoy!