Custard is an essential ingredient to many desserts and pastries. Having the right consistency, however, can make or break your dish. Custard is made from egg yolks, milk, sugar, salt, vanilla and occasionally cornstarch. Once mixed together, the liquids must be heated over low heat; if the heat is too high, or the length of time too short, the custard will be runny with a milk-like consistency. Good custard should be thick and full with a texture closer to that of yogurt.
Optional: use cornstarch in your recipe. This may help thicken your custard, but also threatens to over-thicken it and turn it into pastry cream, which is not the same as custard. If you choose to add cornstarch it should be added at the beginning of the recipe to dry ingredients.
Heat mixed ingredients in a sauce pan over low heat. The egg proteins will bind to the milk between the temperatures of 149 degrees F and 158 degrees F. Adjust the heat of the stove top burner accordingly to ensure the custard stays between these temperatures.
Optional: Scald your whole milk according to the recipe prior to adding it to the dry ingredients. This prepares the proteins and is important for the thickening of the custard. You can experiment with both ways to see what you prefer.
Stir the custard with the whisk constantly while heating. This ensures that all proteins have available material to bind to and do not stagnate over the heat, which can cause burning or curdling. This also helps to avoid the development of a skin over the custard.
Keep stirring until the custard thickens to a preferred consistency. This will take 10-15 minutes. Use the timer to gauge when it is an appropriate time to pay closer attention to the consistency of the custard. Too little time over heat will result in runny, milky custard.
Remove the custard from heat when it is the correct consistency, or sticking to your whisk. Cover with plastic wrap to prevent a skin from forming and leave to cool on the counter. The custard will thicken further as it cools.