Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang to easily lift the bars out later.
Gently pulse the old-fashioned oats in a food processor a few times until they're broken into smaller bits, but not powdered into flour. This gives a lovely texture! Set them aside.
In a medium bowl, whisk together your prepped oats, all-purpose flour, baking soda, cinnamon, and salt. This ensures all the wholesome dry ingredients are evenly distributed.
In a separate large bowl, whisk the slightly cooled melted butter with both the granulated and light brown sugars for about a minute until well blended. Then, stir in the egg, vanilla extract, and molasses until everything is smooth and fragrant.
Gently fold the dry oat mixture into the wet ingredients using a spatula or wooden spoon. Mix just until the flour streaks disappear – don't overmix to keep the bars tender!
Press the dough evenly into your prepared baking pan. Bake for 20 to 24 minutes, or until the edges look set and the top is a beautiful golden brown. Be careful not to overbake, as this keeps them wonderfully moist and chewy!
Remove the pan from the oven and let the bars cool completely on a wire rack for 1 to 2 hours. This step is key – the bars must be at room temperature before icing.
While the bars cool, whisk together the sifted powdered sugar, milk, and vanilla extract in a medium bowl until you have a smooth, pourable but not too thin icing. Adjust milk slightly if needed.
Once the bars are fully cool, use a spoon to drizzle and spread the vanilla icing over the tops. Let the icing set for about 15-20 minutes, then slice into squares and savor these wholesome, easy-to-make treats!