You can substitute orange extract for orange peel in equal parts by volume without changing the recipe. For example, if your cake calls for 1 teaspoon of orange zest, add 1 teaspoon of extract to the wet ingredients. If you're making frosting, add the extract after you whip the butter. You can also substitute 1 teaspoon of orange liqueur for 1 teaspoon of orange peel.
Making Your Own Extract
You can substitute orange zest for orange extract with the addition of a liquid, or make your own extract with alcohol. Mince enough orange zest to make at least 1/4 cup and place it in a glass jar. Add just enough triple-distilled vodka to cover the zest and lid the jar. Let the alcohol extract the oil from the zest for two weeks; shake the jar every day. Pour the extract through a sieve lined with cheesecloth and store it in a jar.
A.J. Andrews' work has appeared in Food and Wine, Fricote and "BBC Good Food." He lives in Europe where he bakes with wild yeast, milks goats for cheese and prepares for the Court of Master Sommeliers level II exam. Andrews received formal training at Le Cordon Bleu.