Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a large mixing bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
Step 3: In another bowl, whisk together sugar, egg, milk, oil, and vanilla until smooth.
Step 4: Gradually add the wet mixture to the dry ingredients, stirring gently until just combined.
Step 5: Carefully stir in boiling water.
Step 6: Drop batter onto lined baking sheets and bake for 10-12 minutes.
Step 7: Beat egg whites until foamy, then gradually add sugar and cream of tartar until stiff peaks form.
Step 8: Melt chocolate with heavy cream over low heat until smooth.
Step 9: Spread marshmallow filling on one cookie and top with another to create a sandwich.
Step 10: Roll edges in graham cracker crumbs and serve!
