Preheat the oven to 350 degrees. Line a cookie sheet with parchment paper. Set aside. In the bowl of a stand mixer fitted with the paddle attachment, on medium to low speed, mix together all of the ingredients except for the butter.

Add in the softened butter and mix on low until everything is combined.

Spread the mixture on the prepped cookie sheet. Bake for about 5 minutes, then with a spatula, stir the mixture around and bake for another 5 minutes. It’s done when the crumble has a light toasted color to it.

Let it cool, then store in an airtight container until you're ready to assemble your cake. It can be stored at room temperature for up 5 days in an airtight container.

Preheat your oven to 325 degrees F. Spray three 8-inch or four 6-inch round cake pans with cooking spray. Spray the bottom of the pans, line the bottoms with parchment paper, and spray again. Set aside.

In a medium sized bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, oil, and sugar on high until fluffy and smooth, about 4 minutes.

With the mixer on medium speed, start adding the whole egg and the egg whites, one at a time, to the bowl, scraping the bottom and the sides of the bowl between each addition. After all the eggs have been added, beat on medium speed for 1 minute. Add the sour cream and vanilla. Beat on medium speed for 30 seconds.

Turn the mixer on low and add half of the flour mixture and half of the buttermilk to the butter mixture and mix until fully incorporated. Finally, add the rest of the flour mixture and the rest of the buttermilk, while mixing on low.

With a rubber spatula, fold in the berries coated in flour.

Using a kitchen scale, evenly distribute the cake batter among the three 8-inch or four 6-inch pans (about 18 ounces in the 8 inch, and 12 ounces in the 6 inch) and bake for 25-30 minutes, or until a toothpick comes out with just a few moist crumbs on it or until it starts to get a little golden brown on top.

Remove cakes from the oven and let them cool in their pans for 15 minutes before taking them out of the pans and placing them on cooling racks.

When the cakes are completely cooled, wrap each cake layer individually with plastic wrap and stick it in the refrigerator or freezer until you’re ready to frost.

If your cakes have a small dome, level them when they are cool, but before you place them in the fridge.

In a small sauce pan over medium heat, combine all the berries, sugar and water. Stir until combined. Let it continue cooking until the berries start to open and it creates a sauce consistency.

Make a slurry by combining the water and the cornstarch. Make sure your berry compote is hot and bubbling, add the slurry (the heat helps activate the corn starch). Stir until it starts to thicken. When you first add the slurry, it will be a light purple. Cook it until it darkens in color. This means you are cooking out the flavor of the corn starch.

Set aside and let it come to room temperature. As it cools down, it will thicken even more. Press plastic wrap on the top to help prevent a skin from forming.

You can put it in the fridge, if you desire, to speed up the cooling process.

In the bowl of a stand mixer fitted with a paddle attachment, cream the butter for about 2 minutes on medium speed. Scape down the sides with a rubber spatula.

Add the sifted powdered sugar and mix on low just until incorporated. Scrape down the sides and bottom of the bowl.

Turn the mixer to medium speed and add the whipping cream, vanilla, cinnamon, and salt.

Turn the mixer to medium-high speed and beat for an additional 5 minutes. The frosting will become lighter in color and texture.

In a small bowl, add two cups of the vanilla buttercream and about ¼ cup of the triple berry compote. Mix with a rubber spatula. Set aside.

Before spreading the frosting on your cake, spend a couple of minutes mixing the buttercream by hand with a wooden spoon or a rubber spatula to push out the air pockets.

Pipe a small dollop of frosting onto the middle of a cake board to act as a glue and hold the cake in place. Place the first cake layer, top side up, on the cake board.

Add half of the triple berry buttercream and evenly apply it to the edges of the cake.

Pipe an extra rim of frosting around the edge of the frosting. This will help support the cake layers and prevent the blueberry filling from spilling out. Spread about a ⅓ cup of the berry filling on the cake layer. Note: Don’t overfill with the compote. You can always serve extra on the side.

Sprinkle about half of the crumble over the berry compote.

Gently place the second cake layer on top of the filling/crumble and repeat the same process for the second cake layer.

Place the final cake layer, top side down, on the second layer of filling.

Apply a small amount of frosting around the entire cake to act as a crumb coat. Freeze the cake for 10 minutes to set the crumb coat.

After the crumbs are locked in, continue to frost and decorate the cake with the rest of the buttercream.