In the bowl of a stand mixer fitted with the paddle attachment, cream butter and brown sugar at medium speed until fluffy (about 3-4 minutes), scraping the sides of the bowl periodically. Add egg and vanilla extract; beat well.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
While the mixer is on low speed, gradually add the flour mixture just until combined. Fold in the oats and raisins. Cover and refrigerate for 1-2 hours.
Preheat the oven to 350°F. Line baking sheets with silicone mats or parchment paper.
Using a 4-teaspoon spring-loaded scoop, drop dough onto prepared pans. Place the cookie dough about 2 inches apart.
Bake until cookies are lightly browned, but still slightly soft, about 10-12 minutes. Let cool on the pans for 3-5 minutes. Remove from the pans and let cool completely on wire racks.
Notes
Use old fashioned rolled oats for the best chewy texture.
Light brown sugar keeps the cookies soft and flavorful.
Chill the dough so the cookies stay thick and chewy.
For plump raisins, soak them in warm water for 5 minutes.
Spoon and level the flour to avoid dry or cakey cookies.
Mix the dough gently so the cookies stay tender.
Bake just until lightly golden around the edges.
Let the cookies cool on the pan so they finish setting.
Freeze unbaked dough balls for quick fresh cookies anytime.
Add nuts, chocolate chips, or dried fruit for easy variations.