1.Preheat oven to 180°C/350°F. Grease 20cm x 30cm rectangular pan; line base and long sides with baking paper, extending paper 5cm over sides.
2.Beat butter, caster sugar and extract in small bowl with electric mixer until combined. Stir in sifted flours. Press mixture evenly over base of pan; using fork, rough the surface.
3.Bake slice about 20 minutes. Cool.
4.Sprinkle gelatine over the water in small heatproof jug; stand jug in small saucepan of simmering water, stir until gelatine dissolves.
5.Reserve 1 tablespoon of the extra water. Stir the remaining extra water and white sugar in medium saucepan over low heat, without boiling, until sugar dissolves; bring to the boil. Boil, uncovered, without stirring, until temperature reaches 116°C (240°F) on a candy thermometer. Reduce heat; simmer, uncovered, 5 minutes, without stirring, regulating heat to maintain temperature at 116°C (240°F). Remove from heat.
6.Blend cornflour with the reserved 1 tablespoon water, gelatine mixture, rosewater and a few drops of food colouring until smooth; stir into sugar syrup. Return to heat; simmer, stirring, about 3 minutes or until mixture is opaque. Strain mixture over slice base, skim any scum from surface; stand 3 hours or overnight at room temperature.
7.Serve dusted with sifted icing sugar.
It’s important to use a candy thermometer for guaranteed success with this recipe. Use a hot wet knife to cut the slice.Note