To make the crust, mix butter, brown sugar, honey, cinnamon, salt, and pecans in a medium bowl. Add flours and stir until just blended. Press the dough evenly over the bottom and up the sides of a 12-inch tart pan with a removable bottom. Generously prick the bottom and sides with the tines of a fork. Bake in the lower third of an oven preheated to 350° F until the crust is pale golden, about 20 to 25 minutes. Cool.
For the ganache, place chopped chocolate in a large measuring glass with a pour spout; set aside. Bring cream to a boil over medium heat. Pour hot cream over the chocolate; without stirring, let the cream-chocolate mixture sit for 2 minutes. After 2 minutes, stir with a rubber spatula until the mixture is smooth and glossy.
Acting quickly, pour ganache into the prepared crust. Carefully pick up the tart pan, tilting and rotating to spread the ganache to the edges as needed. Refrigerate uncovered while you prepare the marshmallow fluff (okay to refrigerate until completely set if preparing ahead of time).
To prepare marshmallow fluff, add 1/4 cup cold water in a small dish. Sprinkle gelatin over and let sit for 5 minutes to soften. Swirl or stir liquid and then heat in the microwave on high for 5 to 10 seconds until completely dissolved. Set aside.
In a small saucepan, bring sugar, remaining 1/4 cup water, and 1/4 cup corn syrup to a boil over medium-high heat, stirring occasionally. Continue cooking until mixture reaches 240° F on a candy or instant-read thermometer.
Combine remaining 1/4 cup corn syrup and the gelatin mixture in the bowl of a standing electric mixer. Mix on low speed to combine with the whisk attachment, and then slowly drizzle in hot sugar syrup, aiming for the side of the bowl rather than the whisk. When all the syrup has been added, increase speed to medium-high and mix for 5 minutes. Add vanilla extract and then increase to the highest speed and continue mixing until marshmallow is opaque, glossy, and tripled in volume, about 5 more minutes.
Immediately transfer marshmallow to a piping bag fitted with a large star tip. Pipe heaping dollops of marshmallow around the tart's outer edge and work your way toward the center, filling in any gaps at the end. Alternatively, spread the marshmallow fluff over the chocolate ganache with a spatula, using the back of a large spoon to create peaks and texture (if going this route, allow ganache to chill completely).
Allow the marshmallow to set for at least 30 minutes before gently toasting with a kitchen torch. If you don't have a kitchen torch, place the tart under the broiler until golden brown, watching carefully so it doesn't over-brown.
Refrigerate for at least 30 minutes before serving for easier slicing. This tart keeps well in the refrigerator for up to two days.