Line an 8x8 pan with foil or wax paper for easy removal and cutting later
In a large mixing bowl, cream the butter and sugar until light and fluffy. Add the vanilla.
On low, mix in the flour and condensed milk, alternating a little at a time until well blended.
Fold in the chocolate chips.
Press the dough into the pan and refrigerate for at least 3 hours, until firm.
For the topping, you can melt the chocolate chips in the microwave and stir in creamy peanut butter before pouring over the top of the dough. Personally, I don't like peanut butter in cookies, so I just melted dark chocolate chips with a little coconut oil and poured that over the top. It's not as smooth and creamy, but it's what I prefer. (More like a candy shell.) Your choice! :)
Chill until set firm again and them slice and serve!