Preheat oven to 325°F (163°C) and grease a 9x13-inch baking dish.
In a large bowl, whisk together cake mix, eggs, milk, butter, baking powder, and sour cream until smooth.
Pour half the batter into the dish and spread evenly.
Warm caramel slightly and drizzle evenly over the batter.
Spoon remaining batter gently on top, spreading to cover the caramel.
Bake for 45–50 minutes, until a toothpick comes out with moist crumbs.
Heat cream until hot (not boiling), then pour over chocolate. Let sit for 1 minute, then stir until smooth.
Let cake cool for 10 minutes, then spread ganache over the top. Add toppings if desired.
Allow ganache to set for 10–15 minutes before slicing and serving warm.