Preheat the oven to 350 degrees. Line the bottom of 2 − 9" round cake pans with parchment paper, spray lightly with cooking spray then set aside.

Cream butter and sugars together until light and fluffy. Add egg and vanilla and beat until well incorporated.

Sift flour, cocoa, baking soda and salt into separate bowl.

Add half of flour mixture and half of milk to butter mixture and mix until just incorporated. Repeat with remaining flour mixture and milk.

Divide batter evenly between prepared cake pans and bake 25−30 minutes, until center springs back when touched and a toothpick comes out clean. Remove from oven and cool completely in pans on a cooling rack.

Prepare filling while cakes are cooling by whipping butter and powdered sugar until completely incorporated and no lumps remain. Add marshmallow fluff and whip until incorporated. Add vanilla, salt and milk, whip until light and fluffy (2−3 minutes).

Remove cooled cakes from pans and remove parchment. Place first cake upside down on cake plate. Place all filling in center of bottom cake and spread evenly toward edges with an angled spatula or butter knife. Place top cake carefully (right side up) on filling, press gently into position.

Slice, serve and enjoy!