Pre-heat oven to 350 degrees.
In a large bowl, whisk together flour, cocoa powder, baking soda, espresso powder, cinnamon, and salt. Set aside.
In the bowl of a stand mixer or in a separate large bowl with a hand mixer, beat together butter and brown sugar on medium speed until light and fluffy, about 2-3 minutes.
Beat the egg into the butter/sugar mixture.
Alternate mixing the flour mixture and buttermilk into the batter, adding the flour in three additions and the milk in two, starting and ending with the flour, and mixing just until just smooth, taking care not to over-mix.
Gently fold ¾ cup crushed Cinnamon Pebbles into the batter.
Spoon about two tablespoons of batter into the cavities of a greased whoopie pie pan. If you don't have a whoopie pie pan, spoon batter in mounds on a parchment paper-lined cookie sheet.
Bake at 350 degrees for 8-10 minutes, until cakes spring back when touched. Let cool in pans for about 5 minutes before moving to wire racks to cool completely.
Once cakes are cooled, spoon about 3 tablespoons of marshmallow buttercream onto one cake and sandwich with another.
Roll edges in remaining ½ cup crushed Cinnamon Pebbles.