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1
To make the custard: In a small bowl, whisk the egg substitute and cornstarch together; set aside.
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2
Put the sugar, cocoa powder, and salt into a medium saucepan.
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3
Whisk in the milk.
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4
Bring to a boil over medium heat, stirring occasionally.
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5
Pour about half of the hot milk mixture into the eggs in a steady stream, whisking constantly to avoid scrambling the eggs.
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6
Pour the mixture back into the saucepan and cook for several minutes, stirring constantly, until the custard is thick enough to coat the back of a spoon.
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7
Pour the custard through a fine-mesh strainer into a medium bowl.
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8
Whisk in the melted chocolate.
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9
Cover the custard with plastic film, poking several holes in the film to allow steam to escape.
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10
Refrigerate for several hours, until it is cold.
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11
Fill the baked tart shells with a scant 13 cup of custard each, spreading it evenly into the shells.
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12
Gently press up on the bottoms of the tart shells to remove the outer pan rings.
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13
Set aside.
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14
To make the marshmallow topping: Put the gelatin into a small bowl, add 1 1/2 tablespoons water, and stir gently.
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15
Set aside for 5 minutes to soften.
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16
In a small nonreactive saucepan (not nonstick), stir the sugar with 1/4 cup water.
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17
Bring the mixture to a boil over medium heat, using a pastry brush dipped into cold water to brush any sugar crystals from the sides of the pan.
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18
Boil for 2 minutes, then remove from the heat and stir in the gelatin until it dissolves completely, at least 1 minute.
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19
While the sugar is boiling, using a standing mixer fitted with the whisk attachment, beat the egg whites on high speed until they turn thick and foamy.
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20
With the mixer running, pour the hot sugar syrup into the egg whites in a slow, steady stream, aiming it between the edge of the bowl and the beater.
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21
Continue beating for 5 minutes longer, or until the bottom of the bowl feels slightly warm to the touch.
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22
Add the vanilla and beat for 1 minute longer.
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23
Without delay, transfer the topping to a piping bag fitted with a standard round tip.
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24
(Alternatively, use a resealable plastic bag: fill, press out the air, seal, and snip off one corner.)
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25
Pipe the topping onto the tarts in a spiral, starting at an outer edge and working your way toward the center.
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26
(If using store-bought topping, which does not pipe well, spoon it onto the tarts and toast it just before serving.)
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27
To finish: Wave a kitchen torch over each tart to toast the topping.
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28
(Alternatively, broil the tarts on a baking sheet in the upper third of the oven for 30 to 45 seconds, watching carefully and removing them as soon as the topping browns.)
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29
To serve: Cut each tart in half, and serve immediately.
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30
Refrigerate leftover tarts made with the homemade topping, covered with plastic film, for up to 2 days after toasting.
