Combine evaporated milk and sugar in a saucepan.

Bring the mixture to a boil over medium heat, stirring constantly to prevent scorching.

Continue boiling for 8 minutes, maintaining a constant stir.

Remove from heat and add the butter and chocolate chips.

Stir until the butter and chocolate chips are completely melted and the mixture is smooth.

If desired, add unsweetened chocolate squares at this stage.

Stir until the unsweetened chocolate is fully incorporated.

Add the vanilla extract.

Stir until the vanilla is evenly distributed.

Pour the fudge mixture into a lightly greased 9 x 13 inch cake pan.

Let the fudge cool completely before cutting into squares.