Preheat oven to 350°F.
In a medium bowl, or the bowl of a food processor, combine the oats, flour, brown sugar, baking powder and salt. Add the butter and blend with a fork or your fingers (or pulse in the food processor) until well combined and crumbly. Press into the bottom of a parchment-lined 8x8-inch pan and bake for 8-10 minutes, until pale golden around the edges.
In the same bowl (no need to wash it) stir together the brown sugar and flour. Add the eggs, butter and vanilla and stir until well blended and smooth. Stir in the raisins and pour over the base.
Bake for 25–30 minutes, until golden and set around the edges but still just slightly jiggly in the middle. Cool completely in the pan on a wire rack before cutting into squares.
Makes 12 bars or 16 squares.