Preheat the oven to 350°F. Line 2 large baking pans with parchment, silicone baking mats, or spray with baking spray. Set aside.
In a medium-size mixing bowl sift together flour, cocoa, baking powder, baking soda and salt
Using an electric mixer, cream together butter, granulated sugar, brown sugar and vanilla until fluffy, about 1-2 minutes. Add the egg, beat until incorporated.
Add the dry ingredients alternately with the milk beginning and ending with dry ingredients. Beat on medium speed just until fully incorporated.
Using a medium ice cream scoop, drop mounds onto baking sheets, leaving at least 2 inches between. Bake for 8-10 minutes or until a toothpick inserted into the center comes back clean. Cool on the pan for 5 minutes then remove to a cooling rack to cool completely.
To make the filling: Using an electric mixer, cream butter with vanilla. Lower the speed of the mixer gradually adding the powdered sugar and 2 Tbsp cream. Once combined, Add marshmallow cream, then beat on high for 2 minutes.
Divide filling evenly among half of the rounds. Top each with a matching size round. Store in an airtight container separated by wax paper for up to 3 days. Bring to room temperature for serving.