While the cake is cooling, whisk together the instant vanilla pudding mix and cold milk in a large bowl until thickened, about 2 minutes.

Poke the Cake:

Once the cake is cool, use the handle of a wooden spoon or a similar tool to poke holes all over the cake, about 1 inch apart.

Fill with Pudding:

Pour the prepared pudding over the cake, spreading it into the holes and across the surface. Use a spatula to ensure the pudding fills the holes. Cover and refrigerate for at least 2 hours, or until the pudding is set.

Make the Chocolate Topping:

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Remove from heat and add the chopped semi-sweet chocolate, butter, and corn syrup (if using). Let it sit for a few minutes, then whisk until smooth and glossy.

Top the Cake:

Pour the chocolate ganache over the chilled cake, spreading it evenly. Allow the ganache to set in the refrigerator for at least 30 minutes.

Serve:

Slice and serve the Boston Cream Pie Poke Cake chilled. Enjoy your creamy, chocolatey treat!