Yield: 12 cupcakes. In the photos, I have tripled this to make 36.
1 cup (125 grams) all-purpose flour
1 cup (110 grams) graham cracker or digestive biscuit crumbs, finely processed to be a powder, from about 7 rectangular crackers
1/4 plus 1/8 teaspoon baking powder
1/4 plus 1/8 teaspoon baking soda
1/2 teaspoon fine sea or table salt
1/4 teaspoon ground cinnamon
1/2 cup (115 grams or 4 ounces) unsalted butter, softened
1/3 cup granulated sugar
1/2 cup dark brown sugar
2 large eggs at room temperature
1 cup buttermilk, well shaken
1/4 pound (4 ounces) milk or semi-sweet chocolate, chopped small or in chips
1/3 cup heavy cream
Pinch or two of salt
2 large egg whites
2/3 cup granulated sugar
1/4 teaspoon cream of tartar
3/4 teaspoon vanilla extract
Make the cupcakes: Heat oven to 350 degrees F. Place cupcake liners in a 12-cup standard tin.
In a medium bowl, combine the flour, graham crumbs, baking powder, baking soda, salt and cinnamon. In a larger bowl, cream the butter and sugars together until light and fluffy with an electric mixer. Add eggs, one at a time, scraping down the bowl between each addition. Add 1/3 of dry ingredients and mix until combined, followed by 1/2 of buttermilk, mixing again. Repeat with 1/3 dry ingredients, last half of buttermilk and remaining dry ingredients.
Divide batter between prepared cups, filling each about 3/4 full. Bake for 20 minutes, or until a toothpick inserted into the center of cakes comes out batter-free. Let cool completely before continuing.
Once cupcakes are fully cool, use a paring knife, grapefruit knife or melon baller to remove a small amount of cake from the center of each cupcake, making room for the chocolate filling.
Make the filling: Place chocolate and salt in a heatproof bowl. Heat cream until simmering then pour it over chocolate, let stand for one minute then stir until smooth. Cool this mixture quickly in the freezer, but you absolutely must check in every 5 minutes (set a timer) to stir the mixture to ensure it doesn’t harden in some spots while staying warm in the center. This takes me about 15 to 20 minutes total. Once cool, it will be thicker but still on the loose side.
Transfer this to a plastic bag with the corner snipped off (hold the end or it will run out!) or a cup with a pouring spout. Pour a small amount into each cupcake belly.
Make the frosting: Combine egg whites, sugar and cream of tartar in a heatproof bowl and place this over a pot of simmering water. Whisk constantly until sugar dissolves and whites are lukewarm to touch, about 3 minutes. Remove bowl from simmering water and use an electric mixer to beat with a whisk attachment on low speed, gradually increasing to high, until stiff, glossy peaks form, about 4 to 7 minutes. Add vanilla and mix to combine.
You can frost your cupcakes with a knife, or drop a dollop of frosting on top messily with a large spoon, or you can use a plastic or piping bag with a larger corner snipped off to make a swirl of frosting on top. (I use a pastry tip with a 1/2-inch round opening.)
To finish: Use a kitchen torch to lightly toast the frosting.