Step 1: Prepare the cake according to package directions. Allow the cake to cool completely in the pan before removing. Once cool, cut the cake into approximately 1-inch cubes. Set aside.
Step 2: In a large bowl, whisk together the instant vanilla pudding mix and cold milk for about 2 minutes, until the mixture starts to thicken. Cover the bowl and refrigerate for at least 15-20 minutes to allow it to set up further.
Step 3: While the pudding chills, prepare the chocolate ganache. Place the chocolate chips in a heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer around the edges (do not boil). Pour the hot cream over the chocolate chips. Let it sit for 5 minutes without stirring to allow the chocolate to melt.
Step 4: After 5 minutes, gently whisk the chocolate and cream together, starting from the center and working your way out, until the mixture is completely smooth and glossy. Allow the ganache to cool slightly to a warm, but not hot, temperature.
Step 5: To assemble the trifle, spoon about one-third of the cake cubes into the bottom of a large trifle bowl (approximately 3-quart capacity) or clear glass dish. Spread half of the prepared vanilla pudding over the cake layer.
Step 6: Drizzle about one-third of the chocolate ganache over the pudding layer. Repeat with another layer of cake cubes, the remaining vanilla pudding, and another third of the chocolate ganache.
Step 7: Finish with the remaining cake cubes as the top layer. Pour the final portion of chocolate ganache evenly over the top layer of cake, allowing some to drip down the sides for a beautiful presentation.
Step 8: Cover the trifle bowl loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to meld and the cake to soften. The longer it chills, the better it tastes!
Step 9: Before serving, you can optionally garnish with whipped cream, chocolate shavings, or fresh berries.