Combine the powdered sugar and tapioca starch in a small bowl.
Line a lightly-oiled 9x12 glass baking dish with parchment or wax paper. Lightly oil the paper and gently dust it with a bit of the powdered sugar mixture. Set the powdered sugar mixture aside for later.
In the bowl of a stand mixer fitted with the whisk attachment, whisk together the powdered gelatin with lemon juice and ¼ cup water. Let sit while you prepare the rest.
In a small saucepan, combine the sugar, light corn syrup, fleur de sel, and remaining ½ cup of water and cook over medium heat, stirring, until the sugar dissolves.
Insert a candy thermometer and increase the heat. Cook, stirring occasionally, until the syrup reaches 240°F.
Turn the mixer to low and gently pour the syrup into the gelatin mixture. Increase the speed to high and whip for about 15 minutes, until the mixture nearly triples in size and becomes white and fluffy.
Add the vanilla bean seeds, vanilla extract, and lemon zest and mix until completely combined.
Pour the marshmallow cream into the prepare baking dish and smooth the top with a spatula.
Leave the pan to dry out, uncovered, overnight.
Dust the top of the marshmallows and a cutting board with the powdered sugar mixture.
Use the parchment paper to help you turn the marshmallows out onto the dusted cutting board. Use a lightly oiled knife to cut the marshmallows into squares.
Toss each marshmallow in the powdered sugar mixture until it's no longer sticky on any side.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.