Preheat oven to 350°F (175°C). Grease and flour (or use a baking spray with flour) a 6-inch round cake pan. Line with parchment paper. Grease the paper.
Whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter, sugar, and brown sugar until light and fluffy. Add the egg and vanilla, and mix well.
Reduce mixer speed to low. Add the flour mixture in 3 portions, alternating with 2 portions of the buttermilk. Mix just until combined.
Transfer the batter to the prepared pan, and spread evenly. Bake 30 to 35 minutes, or until a pick inserted into the center comes out clean.
Cool the cake in the pan on a wire rack for 15 minutes. Then transfer the cake from the pan to a wire rack to cool completely.
To make the frosting:
Using an electric mixer on medium speed, beat the butter, confectioners' sugar, vanilla, and marshmallow creme until thoroughly combined and smooth.
Spread the frosting on the cooled cake. Top with chocolate sprinkles.
Notes
Store in an airtight container at room temperature for 2 or 3 days.