Preheat the oven to 350℉. Spray two 8-inch round pans with nonstick baking spray with flour.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder and salt.
Add the espresso powder, eggs, 1/4 cup of the buttermilk, butter and vanilla to the flour mixture. Using an electric mixer, beat for 1 minute on medium speed, until blended. Scrape the sides and bottom of the bowl with a spatula. Beat on high speed for 2 minutes. Add the remaining buttermilk and beat on low speed for 15 to 30 seconds, until just blended.
Spread the batter evenly in the prepared pans, dividing equally.
Bake for 32 to 37 minutes or until a toothpick inserted into the center comes out with a few moist crumbs attached. Let cool in pans on a wire rack for 10 minutes. Run a knife around the edge of the pans, then invert the cakes onto a rack to cool completely.
MAKE THE FROSTING:
Place the chocolate chips in a large microwave-safe bowl and microwave on medium power for 1 to 2 minutes, stopping every 30 seconds to stir, until melted and smooth. Let cool to room temperature.
Add the marshmallow creme and butter to the cooled chocolate. Using an electric mixer on medium speed, beat until fluffy. Add the powdered sugar and vanilla; beat until smooth and fluffy. Cover and refrigerate for 20 minutes or until firm enough to spread.
ASSEMBLE THE CAKE:
Place one cake layer, flat side up, on a cake plate or platter. Spread some of the frosting evenly over the bottom layer. Top with the second cake layer, flat side down. Spread the remaining frosting over the top and sides of the cake, using the back of a spoon or long, flat spatula to make swirls and peaks on top. Refrigerate for at least 1 hour before serving.