I have a recipe for a very fluffy filling that I use for Whoopie Pies. It is also great as a filling for a cake and I have frosted a cake with it as well.
1 Cup milk
1/4 Cup flour
1 Cup butter or shortening
3/4 to 1 Cup powdered sugar
1 tsp vanilla or other flavoring
In a small sauce pan combine milk and flour. Mix until smooth and no lumps remain. Over medium heat cook until mixture is thick. Cool completely. Place a piece of plastic wrap directly on the mixture so a skin does not form.
In bowl of mixer beat butter or shortening for at least 5 minutes so it is light and fluffy.
Slowly add the sugar and beat well.
Add the cold flour, milk mixture by the spoon full. Beat well between additions of the mixture.
If you wish to make this chocolate add 4 ounces of chocolate chips to the hot milk flour mixture and stir until chocolate has melted.
Any filling recipe for a Whoopie Pie will give you the light fluffy texture you are probably looking for.