Mix and bake the cake according to package directions. Bake it in a 9x13 inch pan. Let the cake cool for one hour.
Meanwhile, add the butter and cream to a small saucepan and set over medium heat. When the mixture comes to a boil, add the bittersweet and semi-sweet chocolate and turn off the heat. Let it sit for a minute or two, then stir until smooth.
Use a knife to cut the cake into 1-inch cubes, and add half the cubes to a mixing bowl. Beat on low until cake is completely crumbly.
Add 1 and 3/4 cup of the ganache to the mixing bowl. Add the remaining cake cubes, and beat until thoroughly combined.
Use a wooden spoon to fold in the chopped strawberries. Try not to eat all of it at this point.
Grab a 9-inch springform pan and line the bottom with a circle of wax paper. Pour or spoon the cake into the pan and smooth the top until it is flat.
Cover with plastic wrap and freeze for 45 minutes. Set a timer. You don't want to actually freeze it, just firm it up. If you can't frost it until later, transfer it to the fridge.
Run a plastic knife around the edge of the pan and gently release the edges of the pan. Invert the cake onto a plate or cake stand and gently remove the wax paper from the now-top.
Carefully frost the top and sides of the cake with the reserved ganache. Yes, the ganache is very thin. It helps if the cake is still very very cold, so you might want to stick it back in the freezer for a few minutes if it's been a while.
Arrange the 6 sliced strawberries on top of the cake.
In a small bowl, combine the white chocolate and oil. Microwave it until the chocolate starts to melt, the stir until smooth. Drizzle the white chocolate over the cake. I added mine to a little plastic bag and snipped a corner.