Preheat the oven to 375 degrees F.
Spread the oats into a single layer on a baking sheet. Bake until lightly toasted, about 20 minutes. Cool the oats for 2 to 3 minutes on the sheet.
Grind 8 ounces of the toasted oats in a food processor until they have the consistency of whole-wheat flour, about 3 minutes. Add the baking powder, cinnamon and salt to the food processor and pulse 2 to 3 times to combine. Set aside.
Line baking sheets with parchment paper.
Combine the butter and sugars in the bowl of a stand mixer and mix on medium speed using the paddle attachment until light in color, about 3 minutes. Stop once to scrape down the sides of the bowl. Reduce the mixer speed to the lowest speed and add the vanilla extract and egg. Mix to combine. Slowly add the flour mixture until just combined. Stop once to scrape down the sides of the bowl. Add the remaining toasted oats and the raisins, if using, and mix to combine.
Scoop the dough with a 1 1/2-tablespoon disher onto the baking sheets, leaving 2 inches between each mound. Bake until the cookies begin to brown around the edges, 12 to 14 minutes, rotating the baking sheets halfway through. Cool on the sheets for 2 minutes, and then move to a cooling rack to cool completely.