1. Bake the Cake: Preheat oven to 350°F (175°C). Grease a 13×9-inch baking pan. Combine cake mix, water, oil, and eggs in a bowl and mix until smooth. Pour batter into the prepared pan and bake for 33-35 minutes, or until a toothpick inserted into the center comes out clean. Cool completely.
2. Poke Holes: Use the handle of a wooden spoon to poke holes all over the cooled cake.
3. Make Pudding: Whisk together pudding mix and milk until smooth.
4. Fill the Holes: Quickly pour and spread the pudding over the cake, filling the holes. Spread any remaining pudding on top.
5. Chill: Refrigerate the cake uncovered for at least 1 hour.
6. Make Ganache: Place chopped chocolate in a heatproof bowl. In a saucepan, combine heavy cream and corn syrup. Heat over medium heat, bring to a simmer, and cook for 3 minutes while whisking.
7. Melt Chocolate: Pour the hot cream mixture over the chocolate. Cover the bowl with plastic wrap and let sit for 5 minutes to melt the chocolate.
8. Whisk Ganache: Uncover and whisk the chocolate and cream mixture until smooth. Let it cool slightly if warm.
9. Frost Cake: Pour and spread the ganache evenly over the cake.

Chill: Refrigerate for at least 3 hours or overnight to set the ganache.