In a bowl, mix sugar and eggs until pale yellow.
When milk mixture is just simmering, SLOWLY add about half of the hot milk mixture to egg yolk mixture. Stir well while mixing.
Combine the resulting mixture back into the pot. Go slowly to avoid scrambling the eggs.
Reduce to medium low heat and stir constantly until the mixture has thickened significantly. The temperature should be about 180F and should look like custard (that's what it is!) .
Strain custard into a bowl, making sure all the barley kernels are strained out, and let cool for about 30 minutes before adding vanilla extract.
Chill custard in refrigerator until it is cold (at least 2 hours).