When Trisha Costello bought the Talkeetna Roadhouse 25 years ago, she was given three things: the deed, a set of keys, and a sourdough starter that the previous owner said dated back to 1902. The year-round population of Talkeetna, Alaska, hovers around 900 people, but in a normal year, Costello’s crew churns out hundreds of pounds of dense sourdough hotcakes per day, each dappled with a cosmos of mixed berries or chocolate chips and hanging over the side of the plate. They’re enormous, and enormously popular, with tourists, with locals,…