Bake cake in 13x9 pan according to package instructions. Allow to cool.
Melt chocolate chips with heavy whipping cream on stovetop over medium low heat. Use a meat injector to insert ⅓ of it into the cake by poking holes and squeezing chocolate into the cake. Alternatively, you can poke holes with a wooden skewer or spoon and pour the chocolate ganache into the holes. The injector allows for a more even distribution.
Next, refrigerate remaining chocolate ganache for about 30 minutes to chill. Pour into a stand mixer bowl and whip at high speed until thickened. The whipped chocolate ganache will be lighter in color.
Spread layer of whipped chocolate ganache over the cake. Refrigerate for 2-4 hours.
Before serving, add a layer of whipped topping and 3 cups of fresh berries.