Measure out 1 cup of raisins in a bowl and cover with warm water. Ideally you’ll want them to sit for 10-15 minutes before draining and adding to the dough.
Using a hand mixer or a stand mixer fitted with a paddle attachment, cream together butter and sugar for 2-4 minutes until light and smooth, occasionally scraping down the sides of the bowl with a rubber spatula as needed.
Beat in 1 egg and 1 tsp vanilla extract for another 1-2 minutes, until well combined. Set aside.
In a separate bowl, sift or whisk together flour, cornstarch, cinnamon, nutmeg, baking soda and salt and a mixing bowl and set aside.
Add in the flour mixture in 2 batches and mix on low speed until just combined.
Add in the oats ½ cup at a time, mixing on the lowest setting until just combined, careful not to overmix.
At this point you can drain the raisins and dry on a paper towel, squeeze out an excess liquid and add to the dough, mix for a few seconds until just incorporated.
Cover the bowl and refrigerate your dough for at least 30 minutes or a few hours. (If you chill overnight be sure to take the cookies out about 30 minutes before baking so you can scoop them more easily.)
After chilling remove the dough from the fridge and preheat the oven to 350 F.
Using a cookie scoop (which I highly recommend because this dough can be a little sticky), scoop balls of dough on a baking sheet lined with parchment paper about 2 inches apart.
Bake at 350 for 9-11 minutes.
Allow cookies to rest on the baking sheet about 5 minutes before transferring them to a wire rack to cool completely. If desired, you can press down on the tops of the cookies to flatten them slightly.
Once they have cooled eat and enjoy! These cookies stay fresh covered at room temperature for about a week or you can freeze the baked cookies for up to 3 months.