PREPARE AND BAKE THE CRUST

1. Preheat the oven to 350°F.
2. Place the graham crackers in a food processor and pulse into very fine crumbs. You will have about 2 cups of crumbs.
3. Transfer the graham cracker crumbs to a medium bowl. Add the granulated sugar and cinnamon and stir to combine.
4. Pour in the melted butter and stir until the crumbs are evenly coated. The mixture should resemble wet sand and hold together when squeezed between your fingers. If it seems too dry or crumbly, stir in additional melted butter, 1 teaspoon at a time, until it reaches the proper consistency.
5. Divide the crumb mixture evenly among six 4-inch mini tart pans with removable bottoms (about 55g per tart).
6. Firmly press the crumbs into the bottoms and up the sides of each tart pan.
7. Place the tart pans on a large baking sheet and bake for 5–7 minutes. If any of the crusts puff up during baking, gently press them back into place while they’re still warm.
8. Transfer the crusts to a wire rack and let them cool completely before filling.

PREPARE AND CHILL THE CHOCOLATE GANACHE

1. Finely chop the dark chocolate into small shards and place it in a medium heatproof bowl with the butter and espresso powder. Set aside.
2. In a small saucepan, whisk together the heavy cream and powdered sugar.
3. Heat the cream over low-medium to medium heat, whisking occasionally, until it just begins to simmer around the edges. Do not let it come to a full boil.
4. Remove the saucepan from the heat and immediately pour the hot cream over the chocolate mixture. Cover the bowl and let it sit for 1–2 minutes.
5. Uncover the bowl and stir until the chocolate and butter are completely melted and the ganache is smooth and glossy.
6. Spoon the ganache evenly into the cooled tart shells and smooth the tops with the back of a spoon or a small offset spatula.
7. Refrigerate the tartlets for 1–2 hours, or until the ganache is set, before adding the marshmallow meringue.

MAKE THE MARSHMALLOW MERINGUE

1. Fill a small saucepan with 1–2 inches of water and bring it to a gentle simmer over medium-low heat.
2. In a medium heatproof bowl, whisk together the egg whites, granulated sugar, and cream of tartar.
3. Place the bowl over the saucepan, making sure the bottom of the bowl does not touch the simmering water.
4. Whisk constantly until the sugar has completely dissolved and the mixture is thin and frothy, about 5 minutes. To check, rub a small amount of the mixture between your fingertips—it should feel completely smooth with no sugar granules remaining.
5. Carefully remove the bowl from the saucepan and add the vanilla extract. Using an electric hand mixer, or a stand mixer fitted with the whisk attachment, beat until the meringue forms stiff, glossy peaks that hold their shape when the beaters are lifted, about 5–7 minutes.

ASSEMBLE THE TARTS

1. Transfer the marshmallow meringue to a piping bag fitted with a large round tip, or simply snip off the end of the piping bag.
2. Remove the tartlets from the refrigerator and carefully release them from the tart pans. Pipe a generous dollop of marshmallow meringue onto the center of each tart. If desired, use a small offset spatula to create soft swirls.
3. Use a kitchen torch to lightly toast the marshmallow meringue until golden brown.

Store the assembled tarts in a covered container in the refrigerator for 2-3 days.