Preheat oven to 350 F (180 C), Line an 8x8" pan with parchment paper.

Combine flours, corn starch, sugar, and xanthan gum in a food processor. Add butter and egg, blitz until finely crumbly.

Spread mixture out in prepared pan, distributing evenly. Firmly press to create a level crust.

Bake for 15 minutes.

In a medium mixing bowl, whisk together brown sugar, corn starch, baking powder and salt.

Add butter, eggs, maple syrup and vanilla, whisk to combine well. Add raisins - and walnuts, if using - stir. Pour mixture over baked crust.

Bake bars for 20 minutes, remove from heat, cool to room temperature.

Once cool, transfer to fridge and chill for an hour before cutting into squares.