Preheat oven to 350 degrees and grease a 9x13-inch baking dish.
In a large bowl, mix together cake mix, eggs, and vegetable oil. Mixture will be thick.
Transfer 2/3 of the batter to the prepared baking dish and press it to completely cover the bottom.
Place condensed milk, chocolate chips, and butter in a microwave-safe bowl. Microwave for 30 seconds and stir. Microwave for another 20 seconds. Stir. If not completely melted, microwave another 20 seconds.
Pour chocolate mixtue evenly into baking dish.Drop the remaining batter by small spoonfuls over the chocolate mixture.
Bake for 20 to 24 minutes. Let cool before cutting into squares.