In a medium saucepan (or microwave-safe bowl), melt the butter and chocolate chips with the sweetened condensed milk. Remove from heat and stir in the flour, pecans and vanilla.
Drop the batter by teaspoonfuls on cookie sheets lined with parchment paper (or sprayed with nonstick spray).
Bake for 7 minutes. Let cool on the pan for a few minutes, then remove cookies to a cooling rack to cool completely.
Notes
These cookies can be kept in a covered container at room temperature for up to 3 days, or they may be frozen.