Grease an 18cm x 28cm (7 inch x 11 inch) rectangular slice pan; line the base and sides with baking paper, extending the paper 2cm (3/4 inch) above the edges of the pan.
Arrange the sponge finger biscuits in a single layer over the base of the pan, trimming to fit.
To make the yoghurt topping, sprinkle the gelatine over the water in a small heatproof jug; stand the jug in a pan of simmering water and stir over the heat until the gelatine has completely dissolved.
Combine the yoghurt, honey and 2 tablespoons of the passionfruit pulp in a medium bowl; stir in gelatine mixture.
Place the egg whites in a small bowl; beat with an electric mixer until firm peaks form; do not overbeat; add the egg whites to the yoghurt mixture in two batches, folding gently to combine; pour over the sponge finger biscuits in the prepared pan; smooth over the top and refrigerate overnight.
Remove the slice from the pan; cut into squares; drizzle with the remaining passionfruit pulp and decorate with mint.