In a large bowl with a hand mixer or a stand mixer fitted with the paddle attachment, beat the softened butter, brown sugar, and granulated sugar together on medium speed until smooth and creamy, about 3 minutes. Add the eggs and continue mixing on high until fully incorporated, around 1 minute. Scrape the sides and bottom of the bowl as needed. Mix in vanilla extract and molasses on high speed until well blended.

In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt. Add this dry mixture to the wet ingredients and mix on low speed just until combined. Add the oats, raisins, and walnuts (if using), and mix on low until everything is thoroughly distributed. The dough will be thick and sticky. Cover and chill the dough in the refrigerator for 30 to 60 minutes. If chilling longer (up to 2 days), let it sit at room temperature for at least 30 minutes before shaping.

Preheat your oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats and set aside.

Scoop out balls of dough, using about 2 tablespoons per cookie, and place them 2 inches apart on the prepared baking sheets. A cookie scoop is helpful for handling the sticky dough. Bake the cookies for 12–14 minutes, until the edges are lightly browned and the centers still appear very soft. Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. The cookies will continue to set as they cool.