Melt chocolate chips, butter, and sweetened condensed milk together over low heat stirring often.
Remove from heat and add flour, vanilla extract, and walnuts (if desired). Stir until just mixed. Do not over mix.
Using a teaspoon drop onto a greased cookie sheet. (I usually use a heaping teaspoon for each cookie). Bake for 8-10 minutes until edges brown slightly.
NOTE: The dough/batter sets up as it sits in the pot and the cookies at the end don't look as good as the cookies in the beginning. Don't stir the dough/batter in the pot once it's all mixed. That makes it worse.
Bake directly on cookie sheets, not silicone mats. Cookies will not brown well if baked on silicone sheets.