Preheat your oven to 350°F (175°C). Lightly grease a 9×13-inch baking pan with butter or nonstick spray. Line the bottom with parchment paper, leaving an overhang on the long sides for easy lifting.

In a large bowl, add the yellow cake mix. Whisk in the melted butter, eggs, 1/2 cup whole milk, and 1 teaspoon vanilla extract until the batter is smooth and thick, 1–2 minutes. It should be spreadable, not runny.

Pour the batter into the prepared pan and spread into an even layer. Bake for 18–22 minutes, or until lightly golden and a toothpick inserted in the center comes out clean or with a few moist crumbs. Cool completely in the pan on a wire rack, about 45–60 minutes.

In a large mixing bowl, whisk together the instant vanilla pudding mixes, 2 1/2 cups cold whole milk, and 1 cup heavy cream or half-and-half for 1–2 minutes, until thick and smooth. Stir in 1 teaspoon vanilla extract and a pinch of fine sea salt. Let stand 5 minutes if it seems loose, then whisk again.

Once the cake base is completely cool, pour the vanilla pudding filling over the cake. Spread gently into an even layer, reaching all corners without pulling up the cake. Cover the pan tightly with plastic wrap, pressing it lightly against the pudding surface, and refrigerate for at least 1 hour, until the pudding is set and no longer jiggly in the center.

Place the chocolate chips or chopped chocolate in a medium heatproof bowl. In a small saucepan, heat 1 cup heavy cream over medium heat until it just begins to simmer around the edges. Pour the hot cream over the chocolate. Add the light corn syrup (if using), 1 teaspoon vanilla extract, and a pinch of salt. Let sit 3–4 minutes, then whisk gently from the center outward until smooth and glossy.

Let the ganache cool for 10–15 minutes, stirring occasionally, until slightly thickened but still pourable and just warm, not hot. Remove the pan from the fridge and peel off the plastic wrap. Pour the ganache over the pudding layer and tilt the pan or use a spatula to spread it into an even layer.

Refrigerate the bars for at least 3 hours, or preferably overnight, until the ganache is set and the dessert is firm enough to slice cleanly.

Use a thin knife to loosen the edges, then lift the slab out using the parchment overhang. Cut into 16–24 bars with a sharp knife, wiping the blade clean between cuts for neat layers. Serve cold or slightly cool.