Line & grease a slice pan (18 x 28cm) with baking paper.
Place sugar, flours & cocoa in a large bowl. Make a well in the centre and add the butter and egg, stir to combine.
Press over base of prepared pan and smooth (I use my fingers for this, it is sticky and might feel like there isn’t enough but just keep pushing it out with your fingers).
Cook in a moderate (180 deg C) oven for about 15 mins.
Cool in pan.
Beat butter and sugar until light and fluffy.
Meanwhile, sprinkle gelatine over the boiling water and stir until dissolved.
While motor operating add the gelatine/water to the mixture and beat until thick and creamy.
Beat in essence and food colouring.
Spread evenly over cooled base.
Place chocolate and oil in a bowl over a pot with simmering water.
Stir until melted and smooth.
Remove and cool for 5 mins.
Spread over filling and stand at room temperature until set.
Cut into squares.