01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing excess to overhang for easy lifting.
02 - In a large mixing bowl, combine melted butter and brown sugar. Stir in eggs and vanilla extract until fully incorporated.
03 - Add rolled oats, all-purpose flour, baking soda, and salt to the wet ingredients. Mix until just combined.
04 - Press two-thirds of the oatmeal mixture evenly into the bottom of the prepared baking pan. Reserve the remaining third for the topping.
05 - In a saucepan over low heat, combine sweetened condensed milk, chocolate chips, and butter. Stir constantly until chocolate is fully melted and mixture is smooth. Remove from heat and mix in vanilla extract and salt.
06 - Spread the chocolate fudge evenly over the oatmeal base in the pan.
07 - Drop spoonfuls of the reserved oatmeal mixture over the fudge layer, gently spreading but leaving areas exposed for a marbled appearance.
08 - Bake for 25 to 30 minutes, or until the topping is golden and the center is just set.
09 - Allow bars to cool completely in the pan on a wire rack, then lift out using parchment overhang and cut into 16 portions.