Preheat the oevn to 350 degrees F.
Cut out parchment to fit the bottom of two 6" round springform pans. Grease the inside of the pans. If you only have one pan, like me, you can bake one layer at a time.
Beat together butter, oil, and sugar. Add the eggs and vanilla extract and beat well until light and fluffy - about a minute.
Sift flour, cocoa, and salt into the bowl and gently fold in with a rubber spatula until just combined. Do not over mix.
Pour half the batter into each pan and smooth the top of the batter, pressing it into the sides of the pan.
Bake for 22-25 minutes or until a knife comes out just barely clean and there is no jiggle to the brownies. Do not over bake.
Let cool for at least 15 minutes and carefully remove from the springforms pans, turnign onto plates. Cool completely before assembling the cake.