1. Preheat oven to 350°F (175°C). Line an 8×8-inch pan with parchment paper or lightly grease it.

2. Melt the butter and let it cool slightly. In a large bowl, mix melted butter, brown sugar, and granulated sugar until smooth.

3. Add egg and vanilla extract to the bowl and stir until glossy.

4. In a separate bowl, whisk flour, baking soda, salt, and cinnamon.

5. Combine the dry ingredients with the wet mixture. Stir just until incorporated.

6. Fold in oats and raisins until evenly distributed.

7. Spread dough evenly in the prepared pan.

8. Bake for 22 to 26 minutes, or until edges are golden and a toothpick comes out mostly clean.

9. Let cool in the pan for at least 20 minutes.

10. Whisk powdered sugar with 1 to 2 tablespoons of milk until smooth and drizzle over cooled bars.

11. Slice into squares and serve.