In a small saucepan, heat 2 cups of milk over medium heat until just boiling. Remove before it boils over and keep warm over a Flame-Tamer or other heat diffusing device while you caramelize the sugar.

Put ⅔ cup of the sugar into a small, heavy saucepan and heat over medium-low heat. At first nothing will happen, then the sugar will rapidly darken and liquefy. At once -- while it is still medium-light rather than dark brown -- pour in a little of the boiled milk, stirring rapidly to start dissolving the sugar. Watch out -- it will splatter up furiously at first. Quickly add the rest of the boiled milk and cook, stirring, over low heat until the caramelized sugar is completely dissolved and the milk pale brown, about 5 minutes. Keep warm while you make the custard.

In a mixing bowl, lightly whisk the egg yolks and add the remaining 2 cups milk, and ⅔ cup sugar. Whisk or stir to combine well and dissolve the sugar without creating too much froth. (Alternatively, process the egg yolks, milk, and sugar together in a blender for 20-30 seconds.) Pour the mixture into a medium-size saucepan and place over very low heat or for still gentler cooking with less danger of curdling use a double boiler top set over simmering water. Stirring constantly with a wooden spoon, 3-5 minutes (or a little longer in a double boiler). Be careful -- too high heat or long cooking will curdle the mixture. Add the caramelized sugar mixture and stir to combine smoothly with the custard. Remove from the heat and let cool completely.