Prepare the oats by pulsing 1 ½ cups of old-fashioned oats in a food processor until they resemble coarse flour. Set aside.

In a medium bowl, whisk together ½ cup melted unsalted butter, ½ cup granulated sugar, and ½ cup light brown sugar until creamy. Add 1 large egg, 1 teaspoon pure vanilla extract, and 2 tablespoons molasses. Mix until smooth.

Fold the prepared oats and 1 cup all-purpose flour into the wet mixture until no dry flour remains. Add a pinch of salt and cinnamon if desired.

Preheat oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper. Spoon dough into the pan and bake for 20–24 minutes until golden brown.

Cool in the pan for 15 minutes, then transfer to a wire rack. Mix 1 cup powdered sugar with a splash of milk to make icing and drizzle over the bars. Let icing set for 30 minutes before slicing.