3/4 cup (1 1/2 sticks) unsalted butter, softened
2 cups packed brown sugar
2 large eggs
1/2 teaspoon salt
2 cups all-purpose flour
1 teaspoon baking powder
1 teaspoon ground cinnamon
2 cups quick-cooking rolled oats
2 teaspoons baking soda
3 tablespoons boiling water

Filling (adapted from “The All-American Dessert Book”)
1 tablespoon plus 1 teaspoon unflavored gelatin (about 1 1/2 packets)
1/3 cup plus 1/4 cup water (divided)
3/4 cup granulated sugar
1/2 cup light corn syrup
2 teaspoons vanilla extract
1 cup powdered sugar
8 ounces vegetable shortening