Combine the granola and 1 cup milk in a blender and puree until almost smooth. Add the remaining 1 1/2 cups milk, the flour and melted butter; blend until smooth. Transfer to a bowl, cover and set aside 1 hour. (You can cover and refrigerate the batter overnight; bring to room temperature before cooking.)
Heat a 10-inch nonstick skillet over medium heat and brush with butter. Add 1/4 cup batter and swirl to coat the bottom of the pan. (If the batter is too thick, add a splash of milk before cooking the next crepe.) Cook the crepe until golden on the bottom, 1 to 2 minutes. Carefully flip and cook until golden on the other side, 30 seconds to 1 minute. Transfer to a parchment-lined baking sheet. Repeat with the remaining batter, brushing the pan with more butter and adjusting the heat as necessary.
Assemble the cake: Set aside 1/4 cup cream cheese for topping. Lay a crepe on a plate and spread with some of the remaining cream cheese. (If the cream cheese is hard to spread, microwave 15 to 20 seconds.) Sprinkle with some of the granola. Top with another crepe and spread with some of the jam. Repeat with the remaining crepes, alternating layers of cream cheese-granola and jam and ending with a crepe.
Combine the reserved 1/4 cup cream cheese, the heavy cream and 1 tablespoon sugar in a small bowl; whisk until smooth and slightly thickened. Toss the plums with the remaining 1 tablespoon sugar in a microwave-safe bowl; microwave 2 minutes. Top the crepe cake with the cream mixture and the plums; sprinkle with granola. Cut into wedges.