[header] How to make lemon sauce [title] Combine all ingredients in a transparent microwave-safe bowl. [step] Preferably it'll have a matching lid if you mean to save any for later. If it doesn't, you'll need to cover the top with a paper towel to keep the liquid from getting all over the place. 
[title] Stir until the cornstarch is evenly dispersed, forming a cloudy mixture. [step] The sugar and butter will not dissolve immediately.