Line a square pan with parchment paper and spray with nonstick cooking spray. Set aside.
With a mixer, beat the butter until fluffy. Do this for about 3-5 minutes. Beat in the brown sugar until well combined. Add your main ingredients: vanilla, milk, flour, and oats to the sugar mixture and mix well. Fold in 1/2 cup of chocolate chips.
Press the batter into the prepared pan and cover with a plastic wrap. Refrigerate for 30 minutes or overnight.
To remove from the pan, lift the parchment paper out of the pan and place the bars on a cutting board. If desired, trim the edges to square up the bars.
Get a sharp knife and cut it into 16 squares. After that, cut each square into a triangle.
Place 2 tablespoons chocolate chips into a sandwich bag and microwave on 50% power for 30 seconds. Knead the bag and return to the microwave for an additional 30 seconds at 50% power kneading until melted. If necessary, return to the microwave one more time until melted.
Snip the tip of the bag and drizzle the melted chocolate onto the cookie bars. Let stand for the chocolate to harden and serve. Enjoy!