- Spread ganache on 2 graham cracker squares and sandwich marshmallow between. Do the same for remaining 5 sandwiches.
- Set all 6 sandwiches on parchment paper and pour ganache over top, allowing some marshmallow to peek through.
- Sprinkle with hickory-smoked sea salt to finish.
For Graham Cracker Squares
- Preheat oven to 350 degrees. Whisk flours, wheat germ, salt, baking soda, cinnamon, and cayenne in medium bowl; set aside.
- In a bowl of an electric mixer fitted with paddle attachment, add butter, brown sugar, and honey. Mix on medium speed until pale and fluffy, 2 to 3 minutes, scraping down sides of bowl as needed. Reduce speed to low. Add the flour mixture and mix until combined.
- Turn out dough onto a floured surface and divide into quarters. Between 2 sheets of parchment paper, roll out each piece into squares a bit larger than 8×8 inches, about 1/8 -inch thick. Using a sharp knife, trim the outermost edges of each square and divide into four 4-inch squares.
- Stack parchment and dough on a baking sheet and chill in freezer until firm, about 20 minutes.
- Remove dough from freezer. Pierce crackers using the tines of a fork. Transfer to large baking sheets lined with parchment paper.
- Bake, rotating halfway through, until dark golden brown, 8 to 9 minutes. Repeat with remaining dough. Let cool on sheet 5 minutes, then transfer crackers to wire racks to cool completely.
- Brush the bottom and sides of a 9-inch square baking pan with vegetable oil. Using a small, fine-mesh sieve, dust the pan generously with confectioners sugar, knocking out any excess.
- Put 1/2 cup water in the bowl of a stand mixer fitted with the whisk attachment. Sprinkle the gelatin into the bowl and stir to combine. Let soften while you make the sugar syrup.
- In a heavy saucepan, combine granulated sugar, corn syrup, salt, and remaining ½ cup water. Place over medium heat and bring to a boil, stirring until sugar is dissolved. Insert a candy thermometer into syrup and continue boiling (if the mixture foams up, turn heat down slightly), without stirring, until thermometer registers 240 degrees.
- Remove saucepan from heat and let stand briefly until bubbles dissipate slightly.
- With mixer on low speed, pour hot sugar syrup into softened gelatin in a thin stream down side of bowl. Gradually increase mixer speed to high and beat until marshmallow is very thick and forms a thick ribbon when whisk is lifted, about 5 minutes. Beat in vanilla.
- Scrape marshmallow into prepared pan and use wet fingertips to spread it evenly and smooth top (it will be very sticky). Let stand, uncovered at room temperature, until surface is no longer sticky and you can gently pull marshmallow away from sides of pan with your fingertips, at least 4 hours to overnight.
- Dust a cutting board with confectioners sugar. Use a rubber spatula to pull sides of the marshmallow from edge of the pan (and to loosen marshmallow from bottom of the pan, if necessary). Invert onto cutting board. Dust the top with confectioners sugar. Lightly coat a long thin knife or a chef’s knife with vegetable oil and dust with confectioners sugar to prevent sticking; continue dusting the knife as necessary. Cut into 4×4-inch squares. Coat marshmallows, one at a time, in confectioners sugar, using a pastry brush to brush off any excess.
- Pour chocolate into a medium-sized mixing bowl.
- Heat cream in a small saucepan over medium heat. Bring just to a boil, watching very carefully because if it boils for a few extra seconds, it will boil out of the pot.
- Pour over chopped chocolate and whisk until smooth. Stir in the rum, if desired. Allow ganache to cool slightly before pouring over s’mores.