Whisk 1 (3.4-ounce) box vanilla instant pudding mix and 1 ¼ cups of the cold heavy cream together in a stand mixer until the pudding mix dissolves.

Attach the whisk attachment and turn the mixer on to medium-low speed.

Slowly pour in 2 cups of the cold heavy cream and 1 (14-ounce) can sweetened condensed milk, and continue to mix until fully incorporated, about 30 seconds.

Increase the speed to medium-high. Beat until defined streaks begin to form and the mixture falls in thick ribbons from the whisk, about 1 minute.

Arrange a single, tight layer of graham crackers in an 8x8-inch baking pan (preferably straight sided), breaking them as needed to fit, 5 to 6 graham cracker sheets.

Spread half of the pudding mixture (about 3 ½ cups) on top.

Repeat with a second layer of graham crackers, then spread with the remaining pudding mixture.

Finish with a final layer of graham crackers.

Microwave ¾ cup semisweet chocolate chips, 3 tablespoons light corn syrup, and the remaining ¾ cup heavy cream in a medium microwave-safe bowl, stirring halfway through, until the chocolate is melted and the ganache is smooth, about 2 minutes total.

Let cool to about room temperature, 10 to 15 minutes.

Pour the ganache over the top graham cracker layer and spread into an even layer.

Refrigerate until the graham crackers soften, at least 8 hours or up to overnight.

When serving, for the cleanest cuts, dip the knife in hot water and wipe clean between each cut.

Serve chilled.