In a large bowl, mix together butter and light brown sugar until light and fluffy (about 3 minutes).  Add vanilla extract and beat until combined.

With the mixer on low speed, alternate mixing in flour and sweetened condensed milk and mix until combined.  Gently fold in mini chocolate chips with spatula.

Line 8x8-inch pan with parchment paper.  Press the cookie dough into the pan (the cookie dough may be sticky, so lightly flour your hands).

Cover the bars with plastic wrap and refrigerate for 3 hours or overnight until firm.

For topping, combine the peanut butter and milk chocolate chips in a microwave-safe bowl and heat until smooth.  Spread the mixture over the cookie dough bars.  Chill the bars for 1 hour or until topping is firm.

Yield: 16 large bars