Mix cake mix, vegetable oil, and eggs together in a bowl using a fork until just blended. Divide mixture in half.
Combine sweetened condensed milk, chocolate chips, and butter in a saucepan over medium heat; cook and stir until melted, 2 to 4 minutes. Remove from heat.
Press half the cake mixture into a 9x13-inch pan; pour chocolate mixture over cake mixture. Take a small handful of remaining cake mixture and flatten between your hands; lay atop chocolate layer. Repeat with remaining cake mixture.
Bake in the preheated oven until topping is lightly browned, about 20 minutes. Cool completely before cutting into bars.