Whisk together the flour, baking soda, cinnamon, and salt in a small bowl. I prefer to freshly grind my cinnamon right before measuring it—the aroma is noticeably more vibrant and gives the cookies a superior flavor compared to pre-ground. Set this mixture aside for now.

In a medium bowl, beat the softened butter with both the granulated sugar and packed brown sugar for 1-2 minutes until the mixture is light, fluffy, and pale in color. This creaming process incorporates air into the butter, which helps the cookies rise properly and develop a tender crumb. The packed brown sugar adds moisture that keeps these small-batch cookies from drying out.

Add the egg yolk and vanilla extract to the creamed butter mixture from Step 2, then beat until fully incorporated and the batter looks smooth. The egg yolk acts as both a binder and an emulsifier, helping to create a cohesive dough with a tender texture.

Add the dry ingredient mixture from Step 1 to the wet ingredients from Step 3, then fold in the oats, raisins, and walnuts until just combined. I like to chop my walnuts fairly fine—about 1/4-inch pieces—so they distribute evenly throughout the cookies and don't create any tough, chunky spots. Be careful not to overmix, which can lead to tough cookies. Drop spoonfuls of dough onto a parchment-lined baking sheet, spacing them about 2 inches apart, then refrigerate for 30 minutes. This chilling step prevents the cookies from spreading too much during baking and helps them hold their shape beautifully.

While the dough chills, preheat your oven to 350°F. Once the chilling time is complete, bake the cookies for 10-14 minutes, until the edges are set and the tops are light golden brown. The centers may look slightly underbaked, which is perfect—they'll continue to firm up as they cool on the sheet for a few minutes before transferring to a wire rack.