Preheat oven to 350 degrees. Butter an 8-inch square baking pan. Line bottom with parchment paper, leaving an overhang on two sides; butter paper.
In a large bowl, whisk together butter, sugars, egg, salt, and cinnamon until smooth. Add flour, oats, and raisins; fold in just until combined.
Spread batter in prepared pan, and bake until a toothpick inserted in center comes out with moist crumbs attached, 25 to 30 minutes. Cool completely in pan. Using paper overhang, lift cake onto a work surface; cut into 16 bars.
To store, keep in an airtight container at room temperature, up to 3 days.