Combine the chocolate pudding mix and the milk in a small saucepan. Cook over medium heat, stirring constantly, until it thickens and bubbles are just beginning to form. Remove from the heat and pour the pudding into a large mixing bowl. Press wax paper against the surface of the pudding and cool to room temperature.

Preheat oven to 350F. Grease a 9x13 baking pan. Remove wax paper from pudding and stir. Add chocolate cake mix to the bowl and stir to combine, ensuring there are no pockets of dry ingredients. Pour batter into prepared pan and smooth into an even layer. Sprinkle top with the chocolate chips and walnuts.

Bake 30 minutes, or until a toothpick inserted comes out clean. Cool completely before serving. Leftovers can be stored in an airtight container or covered tightly with plastic wrap at room temperature for up to 3 days.