Heat oven to 375ºF. Line two half sheet pans with parchment paper and set aside.
Spread oats into a single layer on a half sheet pan and bake until lightly toasted, about 20 minutes. Cool the oats on the pan for 2 to 3 minutes.
Grind half the toasted oats in a food processor until they reach 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.
Combine the butter and sugars in the bowl of a stand mixer fitted with the paddle attachment, and mix on medium speed 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 egg and vanilla extract. Mix to combine. Slowly add the ground oats mixture until just combined. Stop once to scrape down the sides of the bowl. Add the remaining toasted oats and the raisins, if using. Stir to combine.
Scoop the dough with a 1 1/2-ounce disher onto the prepared half sheet pans, leaving 2 inches between each mound. Bake until the cookies begin to brown around the edges, 12 to 14 minutes, rotating the pan halfway through cooking. Cool on the pans for 2 minutes, and then move to a wire rack to cool completely.