Preheat oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. Line a 2nd 12-count muffin pan with 6 liners. (This recipe yields about 18).

Using a food processor, grind the graham crackers, to yield about 1 1/2 cups crumbs. Pour into a medium bowl and stir in sugar and melted butter until combined. (You can also pulse it all together in the food processor.) Spoon 1 and 1/2 Tablespoons of crust into each cupcake liner. Press down tightly to create a tight and firm crust. Set aside. (No need to pre-bake.)

Melt the butter and chocolate together in a large heat-proof bowl. I microwave in 30-second increments, stopping and stirring after each until melted and smooth. Whisk in the granulated sugar. Allow to slightly cool for 5 minutes before adding the eggs. (So as not to cook the eggs!) Whisk in the eggs, vanilla, flour, cocoa powder, and salt. Batter will be thick. Spoon evenly into each cupcake liner on top of the graham cracker crusts, filling almost to the top of the liners.

Bake brownie cups for 25–28 minutes or until a toothpick inserted in the center comes out with only a few moist crumbs (no wet batter). Keep your eye on them and check after 22 minutes or so. Mine take around 26 minutes.

Remove from the oven and allow to cool completely in pans.

Place egg whites, sugar, and cream of tartar in a heatproof bowl. Set bowl over a saucepan filled with 2 inches of simmering water. Do not let the bowl touch the water. (You can use a double boiler if you have one.) Whisk constantly until sugar is dissolved, about 4 minutes. The mixture’s temperature taken with an instant read thermometer should be 160°F (71°C). If it’s not, keep cooking and whisking until thinned out and temperature is warm enough. Remove from heat. Add the vanilla extract, then, using a handheld or stand mixer fitted with a whisk attachment, beat on high speed for 5 minutes until stiff, glossy peaks form. You can read more about and watch me make this in my separate marshmallow meringue frosting post.

Pipe onto cooled brownie cupcakes. I used an Ateco 808 piping tip. If desired, use a kitchen torch to lightly toast.

Cover and store leftover cupcakes at room temperature or in the refrigerator for 1–2 days.