Place semi-sweet and/or milk chocolate chips and condensed milk in a small saucepan. Heat on low, stirring often, until everything has melted and the mix has thickened. Remove it from the burner and pour in the peppermint extract and stir. Let the pot cool until about room temperature.
The cookie dough:
Preheat oven to 350 degrees. Prep a 9x13-inch pan with parchment paper (the combination aluminum foil/parchment paper works well here) and lightly butter or use nonstick spray. Have paper overhang the sides for easy lifting later.
Place the butter and sugars in a large bowl. Mix until smooth and creamy. Add the eggs and vanilla.
In a separate bowl, whisk together the dry ingredients: flour, oats, baking powder, baking soda and salt. Mix with the butter mixture. Add mint chocolate chips last.
Place about half the cookie dough in the prepared pan. It will be sticky, but drop it in dollops and smooth it to the edge as best you can. With a solid layer on the bottom, now dollop the cooled chocolate from the saucepan. Use an offset spatula to smooth it to the edges. Plop the rest of the cookie dough on top and don't worry if some chocolate shows through. The dough will ooze out and even as it bakes.
Bake until lightly browned, about 20 to 25 minutes. Set aside to cool for a few hours, then place in fridge to completely set. Cut into bars. Store in sealed container in fridge or on counter.