1) Preheat the oven to 350°F. Grease a 8"x8" pan. Line with foil or parchment, and grease again, if desired (see tips, below left).
2) In a large mixing bowl, beat together the butter and sugar; beat in the eggs one at a time, stopping to scrape the sides and bottom of the bowl between additions. Beat in the vanilla.
3) In a separate bowl, whisk together the flour, baking powder, and salt. Mix into the butter mixture until evenly combined.
4) Stir in the oats and raisins. Pour the batter into the prepared pan, and smooth out the top.
5) Bake for 25 to 28 minutes, until the top is golden brown and the edges just begin to pull away from the side of the pan. The center should be set, not wobbly. Remove from the oven and cool on a rack.
6) Once the bars are cooled to lukewarm, you can use the parchment or foil to lift them out of the pan onto a cutting board. Cut when completely cool.
Adapted from King Arthur Flour - http://www.kingarthurflour.com/recipes/granola-brownie-bars-recipe