Add cream, milk and a splash of vanilla to a saucepan on medium heat. Bring the liquid to a gentle simmer.
While the cream is warming up, whisk egg yolks and sugar together in a bowl. Make sure they’re well combined.
When the cream just starts to simmer, slowly temper it into the yolks while whisking. Pour the mixture back into the saucepan and turn the heat down to medium low. Stir for several minutes until the mixture has thickened slightly and coats the back of a wood spoon or rubber spatula.
Remove from the heat and add 2 whole graham crackers. Allow the ice cream base to steep for 45 minutes, then strain the liquid through a fine mesh strainer. Discard the graham crackers. Allow the ice cream base to chill thoroughly in the refrigerator for several hours or overnight.
Crush remaining 4 graham crackers into crumbs and stir in the cinnamon.
Prepare ice cream in an ice cream machine according to manufacturer’s instructions. Once the ice cream is almost fully set, slowly add in the graham cracker crumbs with the machine running and allow the mixture to evenly combine.
Allow the ice cream to finish freezing in the freezer.
Prepare the hot fudge:
Heat bittersweet chocolate in a heatproof bowl set over a pot of simmering water, stirring frequently until smooth. Remove the pan from the heat and whisk in the cocoa powder.
In a saucepan over medium heat, gently stir together the brown rice syrup, sugar and heavy cream. Allow the ingredients to come to a simmer and then remove from the heat and whisk in the butter and vanilla. Allow the mixture to cool for a minute or two and then whisk in the melted chocolate.
Prepare the marshmallows:
Combine the powdered sugar and cornstarch. Set aside.
Spray a 9×13 baking dish with baking spray and line the bottom with parchment paper, allowing the paper to hang over two sides. Lightly spray the top of the parchment paper.
Soak the gelatin sheets in a large bowl filled with ice water and set aside.
Add 1/2 cup of corn syrup to a stand mixer with the whisk attachment.
In a medium saucepan, gently stir together the sugar, 1/2 cup corn syrup and 1/2 cup water. Turn the heat to medium high and attach a candy thermometer to the side of the pan.
Bring an inch of water to a simmer in a small saucepan. Drain the water from the bowl of gelatin and gently squeeze the softened sheets to remove excess water. Place the bowl on top of the saucepan and allow the gelatin to melt while stirring with a heatproof spatula. Add the melted gelatin to the stand mixer and turn the speed on low.
When the sugar mixture reaches the soft ball stage (235-240 degrees F), remove from the heat and carefully pour the hot syrup down the side of the mixing bowl into the gelatin. When all of the sugar has been added, turn the mixer up to high and allow it to mix for 5 minutes. Turn the speed down to low, add the vanilla and salt. Mix until evenly combined.
Immediately pour the marshmallow fluff into the prepared pan. Allow the marshmallows to cool for 6 hours or overnight.
Dust the top of the marshmallow with the powdered sugar and cornstarch mixture. Using the sides of the parchment, remove the marshmallow from the pan and place it on a cutting board.
Use a pizza cutter to create 2 inch by 2 inch marshmallows (or whatever size you prefer). As you cut them, dust the sides with the cornstarch mixture to prevent sticking. Store these homemade marshmallows in an airtight container for up to 1 week.
Assemble the sundae:
Layer scoops of graham cracker ice cream with hot fudge.
Top with marshmallows and use a culinary torch to flambe the marshmallows before serving.
Notes
All three recipes can be prepared several days in advance of assembling the sundae. The sundae can be simplified by using store-bought hot fudge and/or marshmallows.When reheating the hot fudge, only heat small batches. Repeated warming can cause the sauce to become grainy over time.Gelatin sheets can be purchased online or at specialty food stores. They create a better final product but gelatin powder may be substituted in the recipe.To use gelatin powder, place the powder to a bowl, add just enough water to cover the gelatin and set aside for several minutes. After adding the corn syrup to the stand mixer, microwave the gelatin for 30 seconds and then add it to the stand mixer on low speed.