Preheat oven to 350°F and grease a 9x13 pan.
Cream the butter and brown sugar in a large bowl (or stand mixer) until light and fluffy.
Add the egg and vanilla and stir until well-combined.
Stir in the flour and oatmeal.
Spread about 2/3 of the mixture into the bottom of the pan; set aside.
In another bowl, combine the sweetened condensed milk, chocolate chips and butter.
Microwave on high in 30 second increments, stirring between each, until the mixture is smooth.
Pour over the dough and spread to the edges.
Add small spoonfuls of the remaining dough over the chocolate layer.
Bake for 25 minutes; let cool before cutting.