If you like sweet, buttery bar cookies, here's a quick and easy addition to your repertoire. These moist and chewy treats will keep in the refrigerator, covered, for up to 2 weeks.
Position a rack in the lower third of the oven and preheat to 325 degrees F. Butter a 9x13-inch baking pan and set aside.
Combine the raisins, sugar, flour, and 1-1/2 cups of water in a medium-size saucepan and place over high heat. Bring the mixture to a boil and then lower the heat to medium. Cook, stirring occasionally, until the mixture is thick, about 30 minutes. Remove from the heat and set aside to cool for 30 minutes.
Place the sugar, oats, flour, and baking soda in a large bowl and whisk to combine. Add the butter and blend in with your fingers until the mixture is crumbly.
Pat half of the topping into the bottom of the prepared pan, pressing firmly to form an even layer. Drop dollops of the raisin filling evenly over the crust and spread carefully with your fingers or a spoon to form an even layer to cover the crust. This is a bit tricky — if you move the raisin filling around too much, it will pull up the oats. Cover with the remaining topping, patting it down lightly so it forms a sheet.
Transfer the pan to the oven and cook until the topping is golden brown and the filling begins to bubble, 30 to 40 minutes. Remove the pan from the oven and place it on a wire rack to cool to room temperature. Cut into bars to serve.