Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.

Prepare the yellow cake according to the package directions, mixing in the water, eggs, and oil. Pour into the greased dish and bake for 30 minutes.

Once the cake is cool, poke holes in the surface with a wooden spoon, about an inch apart.

In a large bowl, whisk together the instant vanilla pudding mix, whole milk, and vanilla extract until smooth. Pour over the poked cake.

Cover the cake with plastic wrap and refrigerate for at least 1 hour to set.

For the ganache, heat the heavy cream in a saucepan until it simmers. Pour over the chocolate chips and let sit for 5 minutes.

Stir the chocolate and cream mixture until smooth. Let cool for 10 minutes, then pour it over the chilled cake.

Cover and refrigerate for at least 4 hours, or overnight if possible, before serving.