It's easiest to make this with a special tarte cake pan: Norpro Nonstick 8.5 Inch Fluted Round Tart Cake Pan It creates a well in the centre of the brownie to hold the lake of chocolate.
Simply prepare a pack-mix brownie recipe as directed.
Bake as directed, but keep an eye on the progress. With the unique pan shape, it may get done sooner or later than the box indicates.
While it's in the oven, prepare the ganache. Ganache is basically a chocolate sauce that has a paste-like consistency when cooled. It's made by heating chocolate, a splash of cream and a dollop of glucose. I used semisweet chips, almond milk and corn syrup.
When the brownie cools slightly, fill the indentation with the scalding chocolate mix.
I garnished it with a bit of melted white baking chocolate. Drag a knife or toothpick through the top to create the swirly design.