Lemon Tart by Josi S. Kilpack

Sadie Hoffmiller is a woman "of a certain age" who lives in Colorado in a small cul-de-sac. One morning she is surprised to see the police pull up to her young neighbor's house and goes to find out what's what. The police aren't thrilled with her presence but she has a key to the house and knows the young single mother, Anne, who lives there with her two-year son, Trevor. Sadie discovers that Anne is baking a lemon tart from the recipe that she'd given her. It soon becomes apparent that Anne has been killed and Trevor is missing. This wasn't the greatest mystery I've ever read but it wasn't the worst. I thought Sadie was rather an idiot most of the time.