Line a large rimmed baking sheet with parchment paper. In a small bowl, whisk the flour, cinnamon, baking soda, and salt until the cinnamon is evenly dispersed and no soda lumps remain. Add the rolled oats and whisk again. Keeping the leavener evenly distributed matters in this small dough because even a concentrated pinch can cause uneven browning or a soapy flavor.
Place the softened butter, brown sugar, and granulated sugar in a medium bowl. Using an electric hand mixer on medium speed, beat for 90 seconds to 2 minutes, scraping the bowl once, until the mixture looks creamy and slightly lighter but not fluffy like cake batter. Add the egg yolk and vanilla, then beat for 30 seconds until smooth, glossy, and fully emulsified with no visible yolk streaks.
Add the oat mixture to the butter mixture and mix on low speed, or fold with a sturdy spatula, just until only a few dry flour streaks remain. Fold in the raisins until evenly distributed and no loose flour remains. The dough should be thick, tacky, and easy to mound, not wet or crumbly. Cover the bowl and refrigerate for 30 minutes so the oats hydrate and the butter firms enough to control spreading.
Set an oven rack in the center and preheat the oven to 175°C, or 350°F, for at least 20 minutes. Divide the chilled dough into eight 44-gram portions using a scale or a firmly packed 2-tablespoon scoop. Roll each portion gently into a ball, place them at least 5 centimeters apart on the prepared sheet, and press each to a thick disc about 2 centimeters tall. The dough should feel cool and firm but should not crack around the edges.
Bake the cookies on the center rack for 11 to 13 minutes, rotating the pan from front to back after 8 minutes. They are ready when the edges are golden and set, the tops look dry around the perimeter, and the centers remain slightly puffed and soft but not shiny or wet. Let them cool on the pan for 10 minutes, where residual heat finishes the centers, then transfer carefully to a wire rack and cool for at least 5 minutes before serving.
