Preheat the oven to 350/177C convection (or 325F/162C conventional) and grease and line three 6-inch (15.24 cm) cake pans. 

Use a stand mixer, fitted with the whisk attachment, to beat the butter, granulated sugar and brown sugar together, on medium-high speed, for 1-2 minutes. Add in the oil and beat for another minute, at full speed.

Add in the eggs, one at time, beating well after each addition for about 30 seconds. Make sure to scrape down the sides of the bowl as needed. Add in the sour cream and vanilla and mix just until they’re combined. 

With the mixer on low speed, add half of the flour mixture to the batter, followed by the milk and the other half of the flour. Beat the mixture on medium speed, just until the flour is fully incorporated, making sure to scrape all the flour off the edge of the bowl.

Separate the batter in half into two separate bowls, there should be about 45 oz total. Add the reserved 1/4 cup (30g) of flour to one half and the cocoa powder to the other half.

Place globs of both batters into the cake pans, distributing the batter evenly among all three. To create the marble effect, use a toothpick or a knife to swirl the batter back and forth a few times.

Bake for about 35 minutes, until the cake springs back when it’s poked or a toothpick inserted comes out clean. Remove the cakes from the pans and allow them to cool completely before frosting.

Place the room temperature butter in the bowl of a stand mixer. Using a whisk attachment, beat the butter on medium-high speed for 1-2 minutes, until the color turns to a pale yellow.

Add the salt and the powdered sugar to the butter and mix on low speed, just until it's combined. Turn the mixer to medium-high speed and beat the buttercream for at least five minutes, until all of the sugar is fully dissolved and the buttercream no longer feels gritty. 

Pour in the heavy whipping cream and vanilla bean paste and beat for another 2 minutes on high speed. If the buttercream is too soft to frost, refrigerate it for 10-15 minutes before frosting.

Place the chocolate chips is a medium size bowl and set it aside.

In a small saucepan, over medium-low heat, bring the cream to a gentle simmer. You want there to be bubbles around the edge of saucepan. If you accidentally boil it (bubbles everywhere), let it cool for a minute.

Pour the cream over the chocolate chips and cover it for 2 minutes. Then use a spatula or fork to gently stir the cream into the chocolate until it's completely melted and smooth. Refrigerate the ganache for 5-10 minutes, just to cool it enough to spread on the cake.

If the cakes are domed, trim the tops off to make them flat.

Use 1/4 of the buttercream to frost the first layer and then use 1/4 of the chocolate ganache on top of the frosting. Repeat for the second layer.

Place the third layer on top, smooth out any buttercream that squeezes out between the layers and refrigerate the cake until it's firm.

Remove the cake from the fridge. Place the rest of the buttercream on top of the cake and use an offset spatula to work it's way down the sides. Use a bench scraper to smooth out the edges and leave the rugged edge at the top of the cake, as a border.

Reheat the chocolate ganache for 10-15 seconds in the microwave, mix it until it's smooth and pour on top of the cake, filling it to the frosting borders. Top with macarons, gold foil, blackberries, a dusting of powdered sugar, etc.