2) Whisk the flour, oatmeal, brown sugar, baking powder, bicarbonate of soda and salt in a large bowl. Stir in the melted butter until combined. Set 125g of the oatmeal mixture aside. Press the remaining oatmeal mixture firmly into the prepared tin.
3) Bake until light golden brown, about 8 minutes. Cool completely. Stir the cashews into the reserved oatmeal mixture.
For the filling:
1) Whisk the flour, brown sugar, espresso powder, cinnamon, chilli powder and salt in a medium bowl. Put the chocolate and butter in large microwavable bowl; microwave on medium power until chocolate is softened, stopping every 30 seconds to stir.
2) Stir until the mixture is melted and smooth. Whisk in the eggs, then stir in the flour mixture. Pour the filling over the cooled crust and sprinkle with the reserved oatmeal mixture.
3) Bake until a cocktail stick inserted into centre comes out with few crumbs attached, 35 to 40 minutes. Cool to room temperature, at least 1 hour. Remove using foil overhang and cut into squares.