Ammonia powder is ammonium bicarbonate, a leavening agent that is a precursor of today's baking powder and baking soda; it must be ground to a powder before using. It is also known as hartshorn, carbonate of ammonia or powdered baking ammonia. It is still called for in some recipes especially for cookies.

I would suggest substituting an equal amount of double acting baking powder for the powdered ammonia (hartshorn) in a recipe. (Your kitchen would stink of ammonia while the cookies baked, but the cookies would not taste or smell of ammonia.) .

Ammonium bicarbonate (also called bicarbonate of ammonia, ammonium hydrogen carbonate, hartshorn, or powdered baking ammonia) is the bicarbonate salt of ammonia. Ammonium bicarbonate is formed as shown above and also by passing carbon dioxide through a solution of the normal compound, when it is deposited as a white powder, which has no smell and is only slightly soluble in water. The aqueous solution of this salt liberates carbon dioxide on exposure to air or on heating, and becomes alkaline in reaction. The aqueous solutions of all the carbonates when boiled undergo decomposition with liberation of carbon dioxide and the substance with which the carbonate ion reacted to form the bicarbonate.

At room temperature ammonium bicarbonate is a white, crystalline powder with a slight odour of ammonia that can dissolve in water to give a mildly alkaline solution. It is however insoluble in acetone and alcohols. Ammonium bicarbonate decomposes at 36 to 60 °C into ammonia, carbon dioxide and water vapor in an endothermic process (as it is with many ammonium salts) and so causes a drop in the temperature of the water. When reacted with acids carbon dioxide is produced, while reactions with alkalis give ammonia.