Preheat the oven to 375 degrees. Spray an 8-inch cake pan with nonstick cooking spray (or grease with shortening). Cut a piece of parchment paper the same size as the bottom of the pan and place the parchment paper circle into the bottom of the greased pan.
Slice the butter into small pieces. Heat butter and chocolate chips in a microwave-safe bowl for 30 seconds. Stir. Heat for an additional 20-30 seconds. Stir until melted and smooth. Whisk in sugar and baking cocoa. Whisk in eggs and vanilla.
Pour the batter into the prepared cake pan and bake for 23-25 minutes. The middle won't look done, but the top will have a slight crust.
Let the cake stand for 5-10 minutes after baking. Run a sharp knife around the edge of the cake then place a platter over the cake and flip over. The cake should pop right out onto the platter. Peel the parchment paper off of the cake. Allow the cake to cool completely on the platter/plate.
Heat the cream in a small pan or in the microwave until hot (just comes to a simmer). Pour the cream over chocolate chips and let stand for 5 minutes. Stir until melted and smooth.
Pour the ganache over the cooled cake. Spread the ganache evenly with a spatula. Let the cake stand until the ganache is set up for at least 2-3 hours before slicing.
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Notes
Tip: for clean slices, wipe the knife off after each cut.Cover the cooled cake with plastic wrap. It can be stored at room temperature for up to 2 days. If you want to store it in the refrigerator it will harden and need to thaw at room temperature for 2-3 hours before serving again.