Position oven rack in center of oven. Preheat oven to 375º. Line 2 baking sheets with parchment paper and set aside.
Sift together the flour, cocoa powder, baking powder, soda and salt onto a sheet of waxed paper.
In a large mixing bowl, beat 7 tablespoons butter and sugar until fluffy using an electric mixer. Add egg and egg white, and beat until incorporated. Add half of flour mixture and half of buttermilk to creamed mixture; beat at medium speed just until blended. Add remaining half of flour and buttermilk and vanilla; beat until blended.
Using an ice cream scoop, drop batter into 14 mounds onto the prepared baking sheets, spacing them 2 inches apart. Or drop batter into 26 mounds by well-rounded tablespoons onto the prepared baking sheets. Bake, one baking sheet at a time, until cakes begin to crack and are firm to the touch, 12 minutes for large cakes and 10 minutes for small cakes. Remove from oven and slide the parchment paper, with the cakes, onto a wire rack to cool completely. Use a thin metal spatula to lift the cookies off the parchment paper.
Spread flat side of half of the cakes generously with Cranberry Cream Filling. Top with the remaining cakes, pressing down lightly to seal them together.
Cranberry Cream Filling
Boil, already reduce once, cranberry juice in a small saucepan until reduced to 2 tablespoons, 5 to 10 minutes. Watch often after juice reduces halfway; it will boil down very quickly from here. Let cool completely.
Beat marshmallow fluff and 1/2 cup butter in a large mixing bowl at medium speed of an electric mixer until smooth and fluffy.
Sift the powdered sugar into the bowl and add the cranberry syrup. Beat on low speed until incorporated; increase speed to medium and beat until fluffy. Makes about 1 1/2 cups.
Alternately, you can put all the ingredients in a food processor and process until smooth and creamy.