Line a square pan with parchment paper or heavy foil.
Coat the paper or foil with cooking spray.
Place the butter and brown sugar in the bowl of a mixer and beat until fluffy. Add the vanilla and beat for one minute. Alternate between adding the flour and condensed milk, mixing slowly until everything is combined and a dough forms. Stir the mini chocolate chips into the dough.
Press the dough into the pan. Cover the bars with saran wrap and refrigerate for 3 hours or overnight until it hardens enough to cut.
Keep chilled until ready to serve, then cut into squares and enjoy. Store any leftovers in the fridge.