How to Make It
Preheat the broiler. Line a large baking sheet with parchment paper and lightly spray the parchment with vegetable oil spray. Spread the Marshmallow Fluff on the parchment in an even 1/2-inch-thick layer. Broil the Fluff for 1 minute, until golden brown all over. Let stand until cool, then fold the fluff a few times to slightly incorporate the browned top.
In a large bowl, using an electric mixer, beat the heavy cream at medium-high speed until it holds firm peaks. Refrigerate the whipped cream until ready to use. In a standing mixer fitted with a whisk, beat the egg yolks at medium-high speed until pale, about 3 minutes. Meanwhile, in a small saucepan, combine the sugar with the water and bring to a boil. Boil over moderately high heat until the syrup reaches 235° on a candy thermometer, about 5 minutes. Remove from the heat immediately.
With the mixer at medium speed, gradually pour the hot sugar syrup into the egg yolks, then beat at medium-high speed until tripled in volume, about 8 minutes. Fold in the chilled whipped cream until no streaks remain.
Using a fork, pull off 1-inch chunks of the marshmallow and fold into the whipped cream. Scrape the semifreddo into an 8-by-11 1/2-inch glass dish, cover with plastic wrap and freeze for 4 hours, until frozen.
Preheat the oven to 325°. Line an 11-by-17-inch rimmed baking sheet with parchment paper. In a standing mixer, beat the butter with the brown sugar and honey at medium speed until pale and fluffy, about 2 minutes. Add the whole wheat flour, 1/2 cup of the all-purpose flour and the salt and mix at low speed until just combined.
Turn the dough out onto the prepared baking sheet and pat it into a disk. Lightly dust the dough with flour and roll it out to a 1/4-inch thickness. Bake for 30 minutes, until golden; turn the baking sheet halfway through for even browning. Let stand until completely cool, about 1 hour, then break into 3-inch shards.
In a small saucepan, bring the brown sugar, corn syrup, heavy cream and cocoa powder to a simmer. Cook over moderately high heat until the sugar is dissolved, about 1 minute. Remove from the heat. Stir in the chocolate chips and vanilla until smooth. Pour the sauce into a small pitcher and let stand until cooled slightly, about 30 minutes. Stir in the salt just before serving.
Let the semifreddo stand at room temperature for 10 minutes. Scoop the semifreddo into bowls and top with the warm chocolate sauce. Garnish with the graham cracker shards and serve.