Preheat your oven to 350°F (175°C).

In a large mixing bowl, cream together softened butter and brown sugar until light and fluffy, about 2-3 minutes.

Add the egg and vanilla extract to the mixture and beat until smooth, about 1-2 minutes.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Gradually add to the wet mixture and stir gently.

Fold in the rolled oats and raisins until evenly combined.

Drop tablespoon-sized mounds of dough onto a non-stick baking sheet, spaced 2 inches apart.

Bake for 12-15 minutes until the edges are golden brown and the centers remain slightly soft.

Let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack.