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Preheat the oven to 350°F with a rack in the center. Line two baking sheets with parchment, or lightly grease.
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In a large bowl working with an electric hand mixer or in the bowl of a stand mixer fitted with the flat beater attachment, beat together the butter, sugars, cinnamon, vanilla, salt, and nutmeg until smooth, pausing once halfway through to scrape the bowl, about 1 minute on medium speed.
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Add the egg and beat until well incorporated, pausing once to scrape the bowl, about 1 minute on medium speed.
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Add the flour and baking powder and mix on low speed to create a smooth, homogenous dough, pausing once to scrape the bowl.
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Add the oats and raisins, and beat on low speed to incorporate fully, pausing once to scrape the bowl.
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Scoop the dough into 1 1/4" portions (about 36g each) onto the prepared baking sheets (a 2-Tablespoon Scoop works well here), spacing the dough balls at least 2" apart. If you’ve added raisins to the oatmeal cookies, use a wet or greased hand to flatten the dough balls to about 1/2" in height to ensure the proper spread during baking.
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Bake the oatmeal cookies one tray at a time for 12 to 14 minutes. For softer cookies, bake for the lesser amount of time; for crunchier cookies, bake for the longer amount. At 12 minutes, a few of the cookies on the edge of the baking sheet should just barely have a bit of a pale brown around their edges. At 14 minutes, the cookies should start to turn brown all over.
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Remove the oatmeal cookies from the oven and let them cool on the pan. Serve slightly warm or room temperature.
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Store leftover oatmeal cookies in an airtight container at room temperature for up to 4 days. Freeze for longer storage.
