For the Shortbread Base:
Get ready: Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper. This makes it easy to lift out later!
Mix it up: In a food processor, combine the cold butter pieces, almond flour, coconut flour, ⅓ cup powdered sweetener, 2 teaspoons gelatin, and 1 teaspoon vanilla extract. Pulse until a smooth dough forms.
Fold in: Gently stir in the ⅔ cup sugar-free chocolate chips.
Press it down: Press the dough evenly into the prepared baking pan with your fingers.
Bake away: Bake for 16-18 minutes, or until the edges are lightly golden. Set aside to cool slightly while you make the marshmallow.
For the Marshmallow Topping:
Gelatin prep: In a small saucepan, pour in 1 cup of water and sprinkle the ¼ cup gelatin over the top. Let it sit for 5 minutes to "bloom" or soften.
Whip whites: In a separate mixing bowl, whip the egg whites on high speed until stiff peaks form. Add the cream of tartar and mix for a few more seconds.
Melt and sweeten: Once the gelatin has bloomed, stir the mixture until smooth. If there are any lumps, place the saucepan over very low heat to melt them, then remove from heat. Stir in ½ cup powdered sweetener until fully combined.
Combine gently: Carefully pour the gelatin mixture into a glass measuring cup with a spout (you can strain it if any tiny lumps remain). With your stand mixer on low speed, slowly drizzle the gelatin mixture into the whipped egg whites.
Whip to fluffy: Once all the gelatin is incorporated, turn the mixer to high speed and whip for 3-4 minutes. The mixture will get thick, shiny, and grow in volume. Stir in the 1 teaspoon vanilla extract.
Layer it up: Quickly spread this marshmallow topping over your baked shortbread base in the pan. Let the bars set for at least an hour, or ideally, overnight, at room temperature.
For the Chocolate Drizzle:
Melt: In a microwave-safe bowl, combine the 8 ounces sugar-free chocolate chips and 2 ounces heavy cream. Microwave for 15-30 seconds, then stir until smooth and melted.
Drizzle and chill: Drizzle the chocolate ganache evenly over the marshmallow layer.
Final set: Chill the bars in the refrigerator for at least 2 hours before cutting into delicious squares and serving.