This is one of the basic dessert sauces. It is luscious simply served with fresh fruit. HEAT THE MILK to the boiling point while you ribbon the yolks with the sugar; beat with a whisk until canary yellow. Then add the flour and beat again. While whisking, pour the boiling milk over the yolk-sugar mixture, then return to the pan. Stirring, preferably with a wooden spoon that has a flat edge to scrape the bottom of the pan, bring just to the boil. Immediately remove from the heat, whisking to cool. Pour through a fine sieve for the best texture. Keeps 2-or-3 days refrigerated. VARIATIONS: CHOCOLATE SAUCE: Add bittersweet chocolate, chopped, to the hot sauce and stir until melted. MOCHA SAUCE: Add 2-to-3 teaspoons of powdered instant espresso coffee. ORANGE SAUCE: Add 2 tablespoons Grand Marnier or Cointreau. Makes 1 1/2 Cups