Preheat the oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners and set aside.

Use a food processor fitted with the blade attachment to finely grind the graham crackers. Add the sugar and melted butter, then pulse until well combined.

Spoon 1-2 tablespoons of the graham cracker mixture into the bottom of each cupcake liner. Use your hands or the flat end of an object to press the crust down firmly, ensuring there are no loose crumbs. Set aside.

Place the chocolate and butter in a heatproof bowl over a double boiler, ensuring the bowl doesn’t touch the water. Simmer over medium heat, stirring often, until the butter and chocolate are melted and smooth. Remove from heat.

Whisk in the sugar and vanilla extract.

Let the chocolate mixture cool for 5-10 minutes, then add the eggs and whisk until combined.

Stir in the flour and sifted cocoa powder, mixing until just combined. Be careful not to overmix.

Spoon the brownie batter on top of the graham cracker crust, filling almost to the top of each liner.

Bake in the center of the oven for 23-25 minutes, or until a toothpick inserted into the center comes out with just a few crumbs. If the center is still wet, continue baking.

Remove from the oven and let the cupcakes cool completely before frosting.

Place the egg whites, brown sugar, and cream of tartar in the metal bowl of a stand mixer.

Whisk to combine, then set the bowl over a pot of simmering water, making sure the bowl doesn't touch the water.

Whisk constantly for 4-5 minutes, until the sugar dissolves and the eggs begin to foam.

Once the eggs are warm to the touch, transfer the bowl to a stand mixer fitted with the whisk attachment. Beat on high speed for 5-6 minutes until stiff peaks form. Add vanilla extract in the final minute of mixing. The mixture should be glossy.

Transfer the meringue to a piping bag fitted with a large star tip and pipe onto the cooled brownie cupcakes.

Note: If making large, tall swirls, you may need to double the frosting recipe to cover all 12 cupcakes.

Use a kitchen torch to toast the frosting, or drizzle with melted chocolate and salted caramel, if desired.