Preheat the oven to 350°F and prepare a 9 × 13-inch baking pan. In a medium bowl, thoroughly whisk together the all-purpose flour, baking soda, ground cinnamon, and fine salt. Mixing the dry ingredients evenly helps distribute the leavening throughout the batter. Set the bowl aside.

Place the softened unsalted butter and packed light brown sugar in a large mixing bowl. Beat for 2–3 minutes, or until the mixture becomes lighter in color and develops a fluffy consistency. Properly softened butter, at approximately 70°F, allows air to be incorporated effectively without making the dough greasy.

Beat in the room-temperature eggs one at a time, making certain the first egg is fully incorporated before adding the second. Pour in the vanilla extract and continue mixing until the wet mixture appears smooth and uniform.

Add the reserved flour mixture to the wet ingredients and stir only until the dry ingredients have been incorporated. Avoid excessive mixing, which can encourage gluten development and produce firmer bars. Fold in the quick oats and raisins with a spatula or wooden spoon until they are evenly dispersed throughout the dough.

Press the dough into the prepared baking pan, smoothing it into an even layer with your fingers or the back of a spatula. Bake for 18–20 minutes, until the surface is golden brown and a toothpick inserted near the center emerges with only a few moist crumbs attached.

Remove the pan from the oven and allow the bars to cool in the pan for 5–10 minutes. They may then be transferred to a wire rack or left in the pan to finish cooling. Allow them to become completely cool before applying the frosting so the butter-based topping does not melt.

While the bars are cooling, beat the softened butter in a clean mixing bowl for approximately 1 minute, until creamy. Gradually add the sifted powdered sugar, beating well between additions to maintain a smooth, airy consistency.

Mix in the maple syrup, cinnamon, and maple extract, if using. Add 1 tablespoon of heavy cream first, then incorporate up to 1 additional tablespoon as necessary to achieve a smooth, spreadable texture. Beat until the frosting is evenly blended.

Once the oatmeal raisin bars are fully cooled, spread or pipe the maple frosting evenly across the surface. Slice into squares with a sharp knife, wiping the blade clean between cuts to create neat edges. Serve immediately or keep the bars in an airtight container.