Prep the pan: Line an 8x8 or 9x9 inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease with coconut oil spray.
Make the crust: In a bowl, mix 2 cups almond flour, 2 tablespoons flaxseed meal, 2 tablespoons granular sweetener, 2 tablespoons brown-style sweetener, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt. Stir in 6 tablespoons melted butter and 1 teaspoon vanilla until it resembles damp sand.
Press and bake: Press crust mixture firmly into the lined pan.
Use a flat-bottomed glass to compress. Bake at 350°F (175°C) for 10–12 minutes, until lightly golden around the edges. Let cool 10 minutes.
Add the chocolate layer: Sprinkle 3/4 to 1 cup sugar-free chocolate chips over the warm crust.
Let sit 2 minutes, then gently spread with an offset spatula to create an even layer. If the crust has cooled too much, briefly place the pan back in the warm oven (turned off) to soften the chocolate.
Bloom the gelatin: In a small bowl, sprinkle 2 1/2 teaspoons unflavored gelatin over 1/4 cup cold water. Let sit 5 minutes.
Make the marshmallow syrup: In a small saucepan, whisk 1/2 cup granular allulose (preferred for smoothness) with 1/4 cup water and a pinch of salt.
Bring to a gentle simmer over medium heat, stirring to dissolve. Remove from heat and whisk in the bloomed gelatin until fully melted. Stir in 1 teaspoon vanilla and a pinch of cream of tartar if using.
Whip the marshmallow: Transfer the hot syrup to a mixing bowl.
Using a hand or stand mixer, beat on high for 6–8 minutes until thick, glossy, and tripled in volume. It should form soft peaks and hold ribbons.
Spread the marshmallow: Quickly spread the marshmallow fluff over the chocolate layer. Work fast; it sets as it cools.
Smooth the top with a greased spatula.
Set and toast: Let the pan sit at room temperature for 45–60 minutes to set. For a classic s’mores finish, briefly toast the top with a kitchen torch until golden. Alternatively, place under a broiler on the top rack for 20–40 seconds, watching closely to prevent burning.
Cut and serve: Lift the slab out using the parchment.
Dust a knife with powdered sweetener and cut into 16 bars. Wipe the blade between cuts for clean edges.