Preheat oven to 350°F. Line two 12-cup muffin tins with paper liners. Pulse graham crackers in food processor until fine crumbs form.
Mix graham cracker crumbs with melted butter and ½ cup sugar. Press 1½ tablespoons into bottom of each liner. Bake crusts for 7-8 minutes until golden. Cool completely.
Whisk together flour, cocoa powder, 2 cups sugar, baking soda, baking powder, and salt in large bowl.
In separate bowl, beat eggs. Add buttermilk, oil, and vanilla. Pour wet ingredients into dry and mix until just combined. Stir in hot coffee.
Spoon batter over cooled crusts, filling halfway. Place one Hershey's chocolate piece in center of each. Top with more batter until two-thirds full.
Bake for 18-22 minutes until toothpick comes out with few moist crumbs. Cool in pan 5 minutes, then transfer to wire rack to cool completely.
For frosting: Combine egg whites, ¾ cup sugar, cream of tartar, and pinch of salt in heatproof bowl. Set over simmering water. Whisk constantly until sugar dissolves and whites reach 140°F, about 4-6 minutes.
Remove from heat. Beat with mixer on low, gradually increasing to high, until stiff glossy peaks form, 5-7 minutes. Add vanilla.
Pipe frosting onto cooled cupcakes. Broil 30-60 seconds until golden brown, watching closely.