In a food processor, grind graham crackers until only fine crumbs remain. Add sugar and melted butter until fully combined. Divide mixture between prepared cupcake liners, adding one small cookie scoop to each. Press down to create an even layer.

In a medium bowl, sift together dry cupcake ingredients.

In a large bowl or stand mixer, cream together butter and sugar. Mix in egg and vanilla. Scrape bowl as needed. Mix in dry ingredients until thoroughly combined. Add sour cream and cooled coffee, mixing until no streaks remain and all ingredients are fully incorporated. Using a large cookie scoop, add one scoop to each prepared liner. Bake for 18-20 minutes or until cupcakes are set and a toothpick inserted comes out clean. Cool in pan 5 minutes, then transfer to a wire rack to cool completely.

In a heatproof bowl over a saucepan with simmering water, whisk together egg whites, sugar and cream of tartar until sugar is dissolved and egg whites are warm to the touch. Remove from heat. Using a hand mixer or stand mixer, beat egg white mixture until stiff peaks form. Add vanilla in last 30 seconds.

Transfer marshmallow frosting to a large piping bag, fit with desired piping tip. Frost cooled cupcakes.

Use a kitchen torch to lightly char the frosting or sprinkle with graham cracker crumbs.