Preheat oven to 350 degrees. Line a 9x9 inch baking pan with aluminum foil and spray lightly with cooking spray.
In a stand mixer, cream the butter, brown sugar, and vanilla. Add the flour and salt and mix to combine. Add about 2 Tablespoons milk. You want to add just enough so that the dough comes together. Beat well.
Take 1 1/2 cups of the batter and set aside in a medium bowl for the frosting later.
For the Cookie Bars: With the remaining dough, add the egg and baking soda and mix until combined thoroughly. Stir in the chocolate chips gently.
Spoon batter into the prepared pan and bake for 18-20 minutes or until top begins to brown. Remove and cool completely.
For the frosting: While the bars are baking finish making the frosting. The batter is stiff at this point so you want to add milk until your desired consistency. Start with 2 tablespoons of milk and add until you like it. I stir in the milk by hand in a bowl but if you wanted to use a mixer you could. Once it's to the consistency that you like, fold in the mini chocolate chips. Keep frosting at room temperature until bars are done cooling.
Frost cooled bars. If you like the frosting thick like I do you may have to work to gently disperse it on the bars. Cut into squares and enjoy.
Spread
Notes
Source: I blended this and this recipe to make this one.