Preheat oven to 350°F. Line a 9x13” pan with foil or parchment paper and spray with cooking spray.
Cream butter, peanut butter, and both sugars in the bowl of an electric mixer fitted with the paddle attachment. Add egg, vanilla, milk, salt, and baking soda. Mix until combined. Slowly add flour and mix until dough comes together.
Press 2/3 of the dough in the bottom of the pan. The dough is very sticky so spray your hands with cooking spray first. It’ll look like you don’t have enough dough but spread it into a thin layer.
Sprinkle the chocolate chips over the top of the bottom layer. Pour the sweetened condensed milk (SCM) over the chocolate chips. Try to give a 1/2 inch border around the edges where the SCM does not touch the edge of the pan. This prevents bubbling edges.
Spoon tablespoon drops of your remaining dough over the top of the SCM layer. Spray your hands again with cooking spray and gently flatten the dough with your fingers. I sort of spread the dough a bit while I was flattening. It won’t completely cover; you’ll still see the filling.
Bake for about 30-36 minutes, until the bars begin to brown on top. Cool completely before slicing. These are gooey when you cut them unless you chill them first. (They taste better gooey!)
Store in an airtight container for up to 3 days. They can be frozen in sealable containers or ziploc bags between layers of paper towels for up to one month.