1. For the crust, preheat oven to 350°F and lightly grease and 8-inch square pan and line with parchment paper so that it comes up the sides of the pan. Toss all the crust ingredients until well-combined and press into the prepared pan. Bake for 12 minutes and cool while preparing the lemon filling.
2. For the filling, reduce the oven to 325°F. Whisk all the filling ingredients together and pour into the cooled crust. Bake for 20 minutes, until set but with a little jiggle in the centre, then cool to room temperature before chilling for at least an hour.
3. For the marshmallow meringue, stir 3 Tbsp of the cold water with the gelatin and the meringue powder in a large bowl or the bowl of a mixer fitted with the whip attachment and set aside.
4. Bring the remaining 2 Tbsp of water, the sugar and the corn syrup to a boil and boil uncovered and without stirring until it reaches a temperature of 240°F on a candy thermometer. While whipping, carefully pour the hot sugar into the bowl with the gelatin and whip this on high speed until tripled in volume and cooled, about 5 minutes.
5. Remove the lemon square from the fridge. Dollop the marshmallow on top of the filling and spread even. Let chill until set.
6. Use a blowtorch to lightly brown the marshmallow and chill until ready to slice and serve.
The squares will keep for up to 3 days refrigerated.