Preheat oven to 350 degrees F. Line a 13x9 baking dish with parchment paper and set it aside.
In a mixing bowl, beat melted butter with dark brown sugar for 2 minutes. Add in eggs and vanilla extract and beat an additional minute, scraping down the sides of the bowl as needed.
To the bowl, add flour, cinnamon, nutmeg, baking soda, kosher salt, and cornstarch. Mix on low just until combined.
Add in the oats and raisins. Mix until blended. Pour batter into the prepared baking dish.
Bake in a preheated oven for 25-28 minutes, until the edges are lightly browned. Remove from the oven and cool completely before cutting into bars.
If desired, sprinkle cooled cookie bars with a light dusting of powdered sugar.
Notes
Oats- Use old fashioned oats for best flavor and texture. Don't swap for quick oats or steel cut oats.
Raisins- For best results, use fresh raisins. Golden raisins are also delicious. For plump raisins, soak in warm water while you make the cookie dough. Drain and fold into the batter.
Storage- keep bars in an airtight container at room temperature for up to one week. Or freeze for up to 3 months.