Place the chocolate chips and condensed milk in a microwave-safe bowl and heat for 40-50 seconds, watching it the whole time--condensed milk can turn to lava if you're not careful! Stir well, and heat again if necessary; the chocolate chips should blend in. Once that happens, add the vanilla extract and stir in. Set aside to cool to room temperature.
For the cookie bars:
Preheat oven to 350-degrees. Line a 9x13-inch pan with foil, overlapping the sides to make it easy to lift out later. Spray foil with nonstick spray or rub with butter.
Place the butter and both sugars in a large bowl. Beat until smooth and creamy. Add the eggs and vanilla.
In a separate bowl, combine the dry ingredients: the flour, oats, baking powder, baking soda and salt. Gradually add to the butter mixture until everything is well incorporated. Stir in the chocolate chips.
Place about half the cookie dough in dollops in the ready pan. Gently smooth it out to cover the bottom. Pour the cooled chocolate over the dough. Top that with the remaining dough in dollops; don't worry about covering the whole surface.
Bake for 20 to 25 minutes, until the top is just golden brown. Cool completely. Use the foil to lift it out of the pan to cut. Store in a sealed container in the fridge, as the middle will soften at room temperature.