Beat 6 ounces of butter with the brown sugar until light; beat in eggs and 2 teaspoons vanilla. Slowly beat in the cake mix and oats until well blended.
In a saucepan, combine the sweetened condensed milk with chocolate chips, a pinch of salt, and 1 tablespoon butter. Cook over medium-low heat, stirring frequently until chocolate chips are melted, and the mixture is smooth.
Stir in the chopped pecans.
Spread about two-thirds of the cake batter in the prepared pan.
Spoon filling over the cake layer and spread to cover.
Pinch pieces of the remaining batter and arrange the pieces evenly over the filling.
Bake for 25 to 30 minutes, until set and golden brown.