1. In a large bowl, cream together the butter, brown sugar, egg and vanilla until smooth. In a separate bowl, whisk the flour, baking soda, cinnamon and salt together. Stir this into the butter/sugar mixture. Stir in the oats, raisins and walnuts, if using them.At this point you can either chill the dough for a bit in the fridge and then scoop it, or scoop the cookies onto a sheet and then chill the whole tray before baking them. You could also bake them right away, if you’re impatient, but I do find that they end up slighly less thick. Either way, heat oven to 350°F (175°C) before you scoop the cookies, so that it’s fully heated when you’re ready to put them in.The cookies should be two inches apart on a parchment-lined baking sheet.
2. Bake them for 10 to 12 minutes (your baking time will vary, depending on your oven and how cold the cookies were going in), taking them out when golden at the edges but still a little undercooked-looking on top.
3. Let them sit on the hot baking sheet for five minutes before transferring them to a rack to cool.With an electric mixer* beat together the butter and powdered sugar until blended, about 2 minutes.
4. Add the maple syrup and continue to beat until smooth, about 1 minute.There are two options for frosting these cookies - spread a thick layer of buttercream across the tops of all the cookies (my preference), or make cookie sandwiches by spreading a thick layer of buttercream on the bottoms of half of the cookies and top with the remaining cookies.