Heat oven to 350°F (175°C). Line an 8x8-inch metal pan with parchment. In a large bowl, whisk together melted butter, brown sugar, and granulated sugar. Add egg and vanilla, whisk until glossy.

Sprinkle flour, baking soda, cinnamon, and salt over the wet mixture. Stir with a spatula just until combined, a little shaggy is fine.

Fold in the oats and raisins until evenly distributed and the oats are coated.

Press the dough into the prepared pan in an even layer. Bake 22 to 28 minutes, until golden and set in the center (a toothpick should have moist crumbs).

Cool completely in the pan on a wire rack, about 1 hour. Lift out with the parchment, slice into squares, and serve.