Preheat the oven to 350F/177C convection (325F/163C conventional) and grease and line three round 8 inch cake pans with baking spray and parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt until they’re evenly distributed.

Place the butter and sugar in a large bowl or the bowl of a stand mixer and use an electric mixer to beat them at full speed for 1-2 minutes. Add the oil and beat for another 1-2 minutes at full speed. 

Add in the eggs, one at a time, beating for at least 30 seconds between each one, until it’s incorporated. 

Add the vanilla and sour cream and mix just until it’s combined.

Add half of the flour mixture, turn the mixer the low speed and stream in the milk. Add the other half of the flour mixture and increase the speed to medium for another 30-60 seconds, just until there are only a few streaks of flour left. 

Use a rubber spatula to scrape the bottom and sides of the bowl and give it one more good mix by hand. 

Distribute the batter evenly among the prepared pans, about 18-19oz in each pan. 

Bake for 30-35 minutes or until the edge of the cake releases from the pan or springs back when you gently press on the tallest part. 

Flip onto a tea towel or a cooling rack and allow to cool to room temperature before frosting. 

Place the dark chocolate in a large bowl (or the bowl of a stand mixer) and place the heavy whipping cream into a small saucepan. 

Heat the cream over medium heat until it comes to a simmer (bubbling all around the edge).

Pour the cream over the dark chocolate and cover the bowl with a lid or plastic wrap for about 5 minutes, so the chocolate melts.

Use a rubber spatula to gently stir the ganache until they're completely smooth.

Cover again and allow it to cool completely to room temperature. You can refrigerate to speed up the process but make sure to stir it every 5-10 minutes so it cools evenly.

Split the chocolate ganache in half. Reserve half of the ganache for the cake filling and place the other half in the bowl of a stand mixer or a large bowl.

Use the whisk attachment of an electric mixer, at full speed, to beat the ganache for 3-5 minutes, until the ganache has lightened in color and grown in volume.

Place the first layer of cake on a flat surface. Place the WHIPPED ganache into a piping bag and pipe a border around the edge of the cake. Fill the center with half of the reserved ganache. Repeat with the next layer and then top with the last layer.

Use the rest of the WHIPPED ganache to frost the outside of the cake. You should have enough to lightly frost the cake.* Refrigerate the cake.

Place the dark chocolate in a large bowl (or the bowl of a stand mixer) and place the heavy cream into a small saucepan. 

Heat the cream over medium heat until it comes to a simmer (bubbling all around the edge).

Pour the cream over the dark chocolate and cover the bowl with a lid or plastic wrap for about 5 minutes, so the chocolate melts.

Use a rubber spatula to gently stir the ganache until they're completely smooth.

Cover again and allow it to cool for about 10 minutes. Pour the ganache on top of chilled cake and spread it out so it drips off the edge.