Preheat your oven to 350 degrees. Butter a 9x9 or 9x13 baking dish then lightly dust it with flour.
Cream the butter, brown sugar, and granulated sugar in a large bowl or stand mixer until light and fluffy, about 2 minutes.
Add the eggs, milk, and vanilla and beat until combined.
Add the flour, rolled oats, baking soda, and salt and stir until mostly mixed using a wooden spoon. Add ¾ cup of both the chocolate chunks and chocolate chips and stir until they are mixed into the batter.
Transfer the batter to the baking dish and gently press the top down so that it is smooth. Top with the reserved ¼ cup of both the chocolate chunks and chocolate chips.
Bake for 20-25 minutes if using a 9x13 inch pan or 25-30 minutes if using a 9x9 inch pan. The edges should be golden and the center not wiggly if you shake the pan. Let the bars cool completely before cutting them into squares.