Death by chocolate snickerdoodle / Sarah Graves.

Enhanced descriptions from Syndetics:

In the fourth Death by Chocolate Mystery by the bestselling author of the Home Repair is Homicide series, Jacobia "Jake" Tiptree and Ellie White are fired up for Eastport, Maine's Annual Cookie Baking Contest. But when a cunning killer and a devastating fire threaten to ravage the quaint island town, Jake and Ellie must dip into another homemade homicide investigation before all they love goes up in smoke...

As co-owners of Eastport's beloved waterfront bakery, The Chocolate Moose, Jake and Ellie know their customers expect them to cream the competition. But they're really just in it for fun, hoping to get Jake's daughter-in-law baking again. Those plans collapse when fearsome local curmudgeon Alvin Carter is murdered, and every crumb of evidence points to Tiptree family friend--and all-around sweet guy--Billy Breyer.

Billy's sisters beg Jake and Ellie to prove his innocence. After all, lots of folks had gone sour on Alvin--the only thing the retired lawyer liked better than bilking widows was swindling orphans, and several victims of his long-ago schemes still lived in Eastport. But just as the ladies begin sifting through the suspects, a series of grass fires blaze across the island, cutting off access to the mainland. Could someone be trying to hide the truth about Alvin's murder?

Now, Jake and Ellie will need all their courage--and an extra dash of that down-east Maine stubbornness--to sniff out the real killer before anyone else gets burned...