Tape a 8" cake board to a 6" one, and place the cake boards onto a cake turntable.
With an offset icing spatula, spread on a little bit of buttercream on the center of the cake board to act as "glue" holding the cake onto it.
Place down the first chilled cake layer. Spread on a thin amount of buttercream, then pipe a dam of buttercream around the outside rim of the cake layer. Add in about 1/2 cup of chocolate ganache filling. Repeat with the second cake layer, then add the final cake layer on top.
Crumb coat, then freeze.
Add on the final coat of buttercream, then lightly use a cake scraper to make clean sides and edges (if the chocolate bits are dragging, either pull out the problem bit, or gently smooth over it with a lighter touch).
Drizzle on a little ganache on top of the cake, then freeze for about 1 minute to set.
Add the reserved white buttercream into a piping bag fitted with a Wilton 1M piping tip, then add on the buttercream swirls around the top of the cake.
Sprinkle on a few of the mini chocolate chips to finish.