Preheat oven to 350°F (180°C).

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar at medium speed until light and fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.

In a medium bowl, whisk together flour, graham cracker crumbs, cocoa, espresso powder, baking powder, and salt. With mixer on low speed, gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating until just combined after each addition. Fold in mini marshmallows.

Spray a 6-cup charlotte cake pan with baking spray with flour. Spread batter into prepared pan.

Bake until a wooden pick inserted in center comes out clean, 35 to 40 minutes. Let cool in pan for 10 minutes. Invert cake onto a wire rack, and let cool completely.

Spread Chocolate Ganache onto cooled cake. Top with graham crackers, chocolate pieces, and marshmallows as desired. Store in an airtight container for up to 2 days.