Preheat the oven to 350°F (postpone this step if you are planning to refrigerate your dough for a couple of days).
Combine the flour, baking soda, cinnamon, and salt in a medium-size bowl.
Blend the butter and both sugars together in a large bowl with a hand mixer (or a standing mixer), until well blended. Beat in the egg, then beat in the vanilla. Add the flour mixture gradually mixing on low speed until each batch is incorporated. Mix in the oats, and the raisins.
Use a ¼ cup measuring cup (or ice cream scoop) to measure out balls of the dough, which should be placed on an ungreased cookie sheet with at least 3 inches in between each ball (you should bake 6 cookies per classic-sized cookie sheet). Use the bottom of a glass to flatten each ball. Cover the baking sheet with plastic wrap and refrigerate for 2 days, longer if you like. (You can also layers the flattened disks of dough two layers deep in a container, with parchment or wax paper in between the layers, if that’s easier in terms of fridge space).
Bake the cookies for about 11 to 16 minutes (see Note! This is important!), until they are golden brown, but still have nice give in the middle. Let them sit on the baking sheet for a couple of minutes before transferring them to a cooling rack.
If your cookie dough is at room temperature, whether you’ve just made it, or you’ve let it return to room temperature before baking, you must start checking the cookies at 11 minutes. They tend to go from almost done to definitely done very quickly. The longer baking time is if you are baking them straight from the fridge. I need to play around with this a bit more, but I do feel like baking them from room temperature is the most successful in terms of texture and consistency. And having said that, the few that got a bit more crisp-cooked were UN-believable crumbled over a bowl of vanilla ice cream. Nothing to be sad about here.