Can Parkay Spray be used in place of melted butter for baking muffins?
Although a single serving of Parkay Spray lists 0 calories on its label that is not true (due to labeling rules they are allowed to state it as such). 


An 8oz bottle of Parkay Spray contains 832 calories.
8oz of butter contain approximately 1625 calories.


Parkay Spray:


  Ingredients: Water, Soybean Oil, Buttermilk, Salt, Contains Less Than 2% of The Following: Soy Lecithin And Polyglycerol Esters of Fatty Acids (Emulsifiers), Xanthan Gum, Potassium Sorbate And Sodium Benzoate (to Preserve Freshness), Lactic Acid (Acidulant), Natural And Artificial Flavor, Vitamin A Palmitate, Colored With Beta Carotene (Source of Vitamin A). Contains Milk, Soy.


With a list of ingredients like that there are better ways to lessen the calorie count of your baked goods. Additionally the water in the spray may have a negative effect on your final product as well.

Many times you can just use less butter/oil. Some recipes replace some or all of the fat with things like unsweetened applesauce (approximately 100 calories for 8oz), canned pumpkin puree (100 calories/8oz) or baby food fruit purees. These kinds of substitutions can add additional nutrients and flavors, making for a tastier finished product.

The internet is a great resource for low/reduced calorie/fat recipes.