Preheat the oven to 350 degrees F / 180 C. Line an 8”x8” square cake tin with baking paper, set aside.
Add enough graham crackers to make 1 cup of finely ground crumbs to a high speed food processor and blend until finely ground.
Add the graham cracker crumbs, butter and light brown sugar to a mixing bowl, mix until well combined.
Firmly press into the prepared cake tin and bake for 10 minutes.
While the crust is cooking, whisk the lemon filling ingredients (white sugar, tapioca flour, gf flour, eggs, lemon juice, lemon zest, vanilla) together until smooth and no lumps.
When the crust has finished cooking and is lightly golden brown, remove from the oven and gently pour the filling over the base.
Reduce the oven temperature to 320 degrees F / 160 C.
Return the lemon bars to the oven and cook for another 20 minutes.
Remove and cool slightly at room temperature before transferring to the fridge to cool completely.
Chill for at least 5 hours or overnight before slicing into 16 squares.
Optional topping: Lightly dust the tops of the chilled lemon bars with a sprinkling of powdered sugar before slicing (if using).
Store lemon bars in an airtight container in the fridge for up to 5 days.