Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats to prevent sticking and over-browning on the bottoms.

Place the raisins in a small bowl and cover with warm water. Let them soak for about 10 minutes while you prepare the dough. Drain well and pat dry with paper towels before adding to the dough.

In a medium bowl, whisk together the flour, baking soda, cinnamon, and salt until evenly combined and free of clumps.

In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar with a stand mixer or hand mixer on medium speed for 2–3 minutes, until light, pale, and fluffy.

Add the eggs one at a time, beating well after each addition and scraping down the sides and bottom of the bowl. Mix in the vanilla extract until the mixture is smooth and emulsified.

Add the flour mixture to the butter mixture. Mix on low speed just until the flour is almost fully incorporated and only a few streaks remain. Do not overmix.

Add the rolled oats, drained raisins, and chopped nuts (if using). Stir by hand with a sturdy spatula or wooden spoon until everything is evenly distributed. The dough will be thick.

Cover the bowl and refrigerate the dough for at least 30–45 minutes, or up to 48 hours. Chilling helps the flour hydrate, firms the butter, and produces thicker, chewier cookies.

Using a cookie scoop or tablespoon, portion about 2 tablespoons (35–40 g) of dough per cookie. Roll lightly into balls and place on the prepared baking sheets, leaving about 2 inches of space between each. For slightly flatter cookies, gently press the tops down.

Bake one sheet at a time on the middle rack for 10–12 minutes, or until the edges are set and lightly golden and the centers still look slightly underbaked. Start checking at 9–10 minutes, as ovens vary.

Let the cookies cool on the baking sheet for 5 minutes to firm up, then transfer to a wire rack to cool completely. Repeat with the remaining dough, keeping unbaked dough in the refrigerator between batches.