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To make the oatmeal cookie bars: Preheat the oven to 350°F with a rack in the center. Line an 8" square pan with a parchment sling and lightly grease.
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In a medium bowl, whisk together the brown sugar, butter, and molasses until well combined. (A dough whisk is a helpful tool here.)
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Add the egg and vanilla and whisk until well combined.
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Add the flour, oats, cinnamon, baking powder, and salt; switch to a flexible spatula and mix until combined and no dry streaks remain.
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Add the raisins; stir until well distributed.
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Transfer the mixture to the prepared pan and spread it into an even layer. (A small offset spatula is a helpful tool here.)
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Bake the oatmeal cookie bars for 25 to 30 minutes, or until evenly browned, set in the center, or a digital thermometer inserted into the center reads 205°F.
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Remove the bars from the oven. Let them cool slightly in the pan before using the parchment to transfer them to a wire rack to cool completely. (The bars can be made 2 days in advance; store airtight at room temperature.)
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To make the glaze: In a medium bowl, whisk together the confectioners’ sugar and 2 teaspoons water to form a thick, shiny mixture. Add additional water, if needed, in 1/2-teaspoon increments until the mixture resembles the consistency of honey and easily drizzles off the whisk.
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Drizzle or pipe the glaze onto the bars. Allow the glaze to set briefly (about 5 to 10 minutes) before slicing.
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Storage information: Store leftover oatmeal cookie bars airtight at room temperature for up to 3 days. Freeze for longer storage.
