Mix sweetened condensed milk and agave in a small saucepan and stir to blend. Add a large pinch of salt. Bring mixture just to a boil, stirring with a wooden spoon until smooth. Add chocolate chips and lower heat to a simmer. Cook, stirring constantly, until chocolate melts completely and mixture is smooth. Stir in walnuts. Pour the warm mixture into an 8 x 8-inch pan and smooth the top with a butter knife. Let cool and refrigerate until cold. Cut into 25 squares.