For Graham Cracker:
- 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 two sheets of parchment paper, roll out each piece into squares a bit larger than 8 x 8 inches, about ⅛-inch thick. Using a sharp knife, trim the outermost edges of each square and divide into 4 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 ½ 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 the sugar is dissolved. Insert a candy thermometer into the syrup and continue boiling (if the mixture foams up, turn the heat down slightly), without stirring, until the thermometer registers 240 degrees.
- Remove the saucepan from the heat and let stand briefly until the bubbles dissipate slightly.
- With the mixer on low speed, pour the hot sugar syrup into the softened gelatin in a thin stream down the side of the bowl. Gradually increase the mixer speed to high and beat until the marshmallow is very thick and forms a thick ribbon when the whisk is lifted, about 5 minutes. Beat in the vanilla.
- Scrape marshmallow into prepared pan and use wet fingertips to spread it evenly and smooth the top (it will be very sticky). Let stand, uncovered at room temperature, until the surface is no longer sticky and you can gently pull the marshmallow away from the sides of the pan with your fingertips, at least 4 hours to overnight.
- Dust a cutting board with confectioners’ sugar. Use a rubber spatula to pull the sides of the marshmallow from the edge of the pan (and to loosen marshmallow from the bottom of the pan, if necessary). Invert onto the 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 the chocolate into a medium-sized mixing bowl. Heat the 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 the chopped chocolate and whisk until smooth. Stir in the rum, if desired. Allow the ganache to cool slightly before pouring over s’mores.
- 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 come marshmallow to peek through.
- Sprinkle with hickory-smoked sea salt to finish.