Ammonium bicarbonate is a leavening agent that decomposes during baking, releasing carbon dioxide gas. This process can enhance the browning of baked goods due to the Maillard reaction, a chemical reaction between amino acids and reducing sugars that gives browned foods their distinctive flavor and color. The decomposition of ammonium bicarbonate at higher temperatures can create a more alkaline environment, which can further promote browning.
In contrast, salt, milk, and corn starch do not have the same direct impact on browning. Salt can enhance flavor and influence texture but does not specifically contribute to browning. Milk can also contribute some browning due to the presence of lactose and proteins, but not to the degree that ammonium bicarbonate does. Corn starch primarily serves as a thickener and does not significantly facilitate the browning process either. Therefore, in this context, ammonium bicarbonate is the ingredient most likely responsible for excessive browning during baking, due to its specific chemical properties and reactions when exposed to heat.