grease a 9x13 inch baking pan and optionally line with parchment leaving overhang
measure ingredients and bring butter to room temperature.
Make batter and bake
cream butter brown sugar and granulated sugar until light and fluffy
beat in eggs one at a time then mix in vanilla
whisk flour baking soda cinnamon and salt then mix into wet just until combined
fold in oats raisins and optional walnuts until evenly distributed
press dough evenly into prepared pan
bake until edges are golden and center is set with moist crumbs 22 to 28 minutes
cool completely in pan then lift out and slice into bars.
Notes
1. For softer bars slightly underbake and cool fully before cutting. Swap raisins for dried cranberries or chopped dates. Add 1 to 2 tbsp milk if dough feels very stiff. Store airtight at room temperature up to 4 days or freeze up to 2 months