Heat oven to 350ºF. Line a 24-count Good Cook Muffin Tin with cupcake liners.
For the Crust: In a large bowl, stir together graham cracker crumbs, 4 Tbsp butter and 1/4 cup sugar. Spoon into the bottom of cupcake liners.
For the Cupcakes: In a stand mixer, beat together 2 cups sugar, 1 1/2 cup softened butter and 2 eggs. Beat until light and fluffy. Add flour, cocoa powder, baking powder, salt, sour cream, water and vanilla. Mix just until smooth. Spoon atop crumb crust in muffin tin. Bake for 18-21 minutes, or until the center of cupcakes spring back when touched lightly. Remove from oven and cool completely.
For the Frosting: In a metal bowl or the metal bowl of a stand mixer, whisk together egg whites, 2 cups sugar and cream of tartar. Fill a large, deep-sided skillet with 2-inches of water. Bring water to a boil, place metal bowl inside skillet. Whisking the egg white mixture constantly, until sugar is dissolved and egg whites are warm. Remove from water bath. With a whisk attachment or electric beater, whip mixture for 5-7 minutes, until mixture is shiny, with stiff peaks.
Spoon mixture immediately into an OodleTips Icing Bag, snip the tip and pipe big swirls of frosting atop cooled cupcakes. Place frosted cupcakes on a baking sheet.
Heat oven to Broil. Place cupcakes on the top oven rack, keeping oven door open, watch closely. Toast just until the tips of the frosting turn light golden brown, about 30 seconds. Remove, press a graham cracker and chocolate square immediately into the center of the cupcake, allow to cool.
Serve within 2 hours for best results.
OPTIONAL CHOCOLATE PUMPKIN TOPPING:
For the Topping: In a microwave-safe bowl, melt chocolate chips and transfer melted chocolate to an OodleTips icing bag. Snip a small portion of the plastic tip, pipe chocolate into pumpkin shapes on parchment paper. Place in freezer until cool.
Just before serving, slide chocolate pumpkins in front of the graham crackers on each cupcake. Note: the pumpkins are very tender to the touch, so we recommend using a pair of tongs to help with the transfer from parchment to cupcake!