Lightly grease a 9x13-inch baking pan with nonstick spray or butter. For easier slicing and lifting, line it with parchment paper, letting the edges hang over the sides like handles.

In a large bowl, prepare the yellow cake mix according to the package directions, using milk instead of water if you’d like a richer crumb. Stir just until the batter is smooth; do not overmix.

Pour the batter into the prepared pan and smooth the top with a spatula. Bake at the temperature listed on the cake mix box (usually 350°F/175°C) for about 22–28 minutes, or until the top is golden and a toothpick inserted in the center comes out clean.

Set the pan on a cooling rack and let the yellow cake base cool fully. If you’re in a hurry, cool it on the counter for about 20 minutes, then place it (uncovered) in the refrigerator for another 20 minutes until completely cool.

In a large mixing bowl, add the instant vanilla pudding mixes, cold whole milk, and cold heavy cream. Whisk for 2–3 minutes, until thick and smooth and the mixture resembles very thick pudding. Stir in the vanilla extract and a pinch of fine sea salt, then let sit for 5 minutes to finish thickening.

Once the cake is completely cool, spread the vanilla pudding filling evenly over the surface. Use an offset spatula or the back of a spoon to nudge it gently into the corners without pressing too hard on the cake base.

Cover the pan lightly with plastic wrap and refrigerate for at least 30–45 minutes, or up to a couple of hours, to allow the pudding layer to set before adding the warm ganache.

Place the semi-sweet chocolate chips in a heat-safe bowl. In a small saucepan, heat the heavy cream over medium-low until it just starts to steam and tiny bubbles appear around the edges; do not let it boil. Pour the hot cream over the chocolate, add the light corn syrup and a pinch of salt, and let sit for 2–3 minutes. Stir gently from the center outward until smooth and glossy, then stir in the vanilla extract.

Let the ganache sit at room temperature for about 10 minutes. It should be warm and pourable, with the texture of warm fudge sauce, but not so hot that it melts the pudding layer.

Remove the pan from the refrigerator and gently pour the ganache over the set pudding layer. Tilt the pan or use a spatula to spread the ganache evenly to the edges, working slowly so you don’t disturb the pudding underneath.

Return the pan to the refrigerator and chill for at least 2 hours, or until the chocolate ganache is set and the bars are firm enough to slice. For the cleanest layers, chill 4 hours or overnight.

If you used parchment, use the overhanging edges to lift the whole slab onto a cutting board. With a long, sharp knife, cut into 16–24 bars, wiping the knife clean between cuts for neat layers. Serve chilled.